diff --git a/CHANGELOG.md b/CHANGELOG.md
index 69d41e7aa..6fd2a6c69 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Changed
+- Using unregistered static class methods in expressions now also triggers a deprecation notice because we will drop support for this in the next major release [#813](https://github.com/smarty-php/smarty/issues/813)
+
 ## [4.4.1] - 2024-02-26
 - Fixed internal release-tooling
 
diff --git a/lexer/smarty_internal_templateparser.y b/lexer/smarty_internal_templateparser.y
index 620498765..ffc85bc06 100644
--- a/lexer/smarty_internal_templateparser.y
+++ b/lexer/smarty_internal_templateparser.y
@@ -785,6 +785,9 @@ value(res)       ::= ns1(c)DOUBLECOLON static_class_access(s). {
         if (isset($this->smarty->registered_classes[c])) {
             res = $this->smarty->registered_classes[c].'::'.s[0].s[1];
         } else {
+            trigger_error('Using unregistered static method "' . c.'::'.s[0] . '" in a template is deprecated and will be ' .
+                    'removed in a future release. Use Smarty::registerClass to explicitly register ' .
+                    'a class for access.', E_USER_DEPRECATED);
             res = c.'::'.s[0].s[1];
         } 
     } else {
diff --git a/libs/sysplugins/smarty_internal_compile_private_modifier.php b/libs/sysplugins/smarty_internal_compile_private_modifier.php
index aea082f01..31fd6e1da 100644
--- a/libs/sysplugins/smarty_internal_compile_private_modifier.php
+++ b/libs/sysplugins/smarty_internal_compile_private_modifier.php
@@ -109,9 +109,11 @@ public function compile($args, Smarty_Internal_TemplateCompilerBase $compiler, $
                             if (!is_object($compiler->smarty->security_policy)
                                 || $compiler->smarty->security_policy->isTrustedPhpModifier($modifier, $compiler)
                             ) {
-                                trigger_error('Using php-function "' . $modifier . '" as a modifier is deprecated and will be ' .
-                                    'removed in a future release. Use Smarty::registerPlugin to explicitly register ' .
-                                    'a custom modifier.', E_USER_DEPRECATED);
+                                if (!in_array($modifier, ['time', 'join', 'is_array', 'in_array'])) {
+                                    trigger_error('Using unregistered function "' . $modifier . '" in a template is deprecated and will be ' .
+                                        'removed in a future release. Use Smarty::registerPlugin to explicitly register ' .
+                                        'a custom modifier.', E_USER_DEPRECATED);
+                                }
                                 $output = "{$modifier}({$params})";
                             }
                             $compiler->known_modifier_type[ $modifier ] = $type;
diff --git a/libs/sysplugins/smarty_internal_templatecompilerbase.php b/libs/sysplugins/smarty_internal_templatecompilerbase.php
index d5c18d31a..10caf5906 100644
--- a/libs/sysplugins/smarty_internal_templatecompilerbase.php
+++ b/libs/sysplugins/smarty_internal_templatecompilerbase.php
@@ -640,7 +640,17 @@ public function compilePHPFunctionCall($name, $parameter)
                         return $func_name . '(' . $parameter[ 0 ] . ')';
                     }
                 } else {
-                    return $name . '(' . implode(',', $parameter) . ')';
+					$first_param = array_shift($parameter);
+					$modifier = array_merge(array($name), $parameter);
+					// Now, compile the function call as a modifier
+					return $this->compileTag(
+						'private_modifier',
+						array(),
+						array(
+							'modifierlist' => array($modifier),
+							'value'        => $first_param
+						)
+					);
                 }
             } else {
                 $this->trigger_template_error("unknown function '{$name}'");
diff --git a/libs/sysplugins/smarty_internal_templateparser.php b/libs/sysplugins/smarty_internal_templateparser.php
index a2dd0d6fb..c37d3c187 100644
--- a/libs/sysplugins/smarty_internal_templateparser.php
+++ b/libs/sysplugins/smarty_internal_templateparser.php
@@ -2425,6 +2425,9 @@ public function yy_r99(){
         if (isset($this->smarty->registered_classes[$this->yystack[$this->yyidx + -2]->minor])) {
             $this->_retvalue = $this->smarty->registered_classes[$this->yystack[$this->yyidx + -2]->minor].'::'.$this->yystack[$this->yyidx + 0]->minor[0].$this->yystack[$this->yyidx + 0]->minor[1];
         } else {
+            trigger_error('Using unregistered static method "' . $this->yystack[$this->yyidx + -2]->minor.'::'.$this->yystack[$this->yyidx + 0]->minor[0] . '" in a template is deprecated and will be ' .
+                'removed in a future release. Use Smarty::registerClass to explicitly register ' .
+                'a class for access.', E_USER_DEPRECATED);
             $this->_retvalue = $this->yystack[$this->yyidx + -2]->minor.'::'.$this->yystack[$this->yyidx + 0]->minor[0].$this->yystack[$this->yyidx + 0]->minor[1];
         } 
     } else {
diff --git a/libs/sysplugins/smarty_security.php b/libs/sysplugins/smarty_security.php
index 97cd0521d..49ae2a386 100644
--- a/libs/sysplugins/smarty_security.php
+++ b/libs/sysplugins/smarty_security.php
@@ -253,7 +253,7 @@ public function __construct($smarty)
      *
      * @param string $function_name
      * @param object $compiler compiler object
-     *
+     * @deprecated
      * @return boolean                 true if function is trusted
      */
     public function isTrustedPhpFunction($function_name, $compiler)
diff --git a/phpunit.xml b/phpunit.xml
index e0ad81d69..15e1d5630 100644
--- a/phpunit.xml
+++ b/phpunit.xml
@@ -15,7 +15,11 @@
         timeoutForSmallTests="1"
         timeoutForMediumTests="10"
         timeoutForLargeTests="60"
-        verbose="false">
+        verbose="false"
+        convertErrorsToExceptions="true"
+        convertNoticesToExceptions="true"
+        convertWarningsToExceptions="true"
+        convertDeprecationsToExceptions="true">
     <testsuites>
         <testsuite name="foo">
             <directory>./tests/UnitTests/</directory>
diff --git a/run-tests-for-all-php-versions.sh b/run-tests-for-all-php-versions.sh
index 79bebb8a6..23541b519 100755
--- a/run-tests-for-all-php-versions.sh
+++ b/run-tests-for-all-php-versions.sh
@@ -5,11 +5,13 @@
 # - ./run-tests-for-all-php-versions.sh --group 20221124
 # - ./run-tests-for-all-php-versions.sh --exclude-group slow
 
-docker-compose run php71 ./run-tests.sh $@ && \
-docker-compose run php72 ./run-tests.sh $@ && \
-docker-compose run php73 ./run-tests.sh $@ && \
-docker-compose run php74 ./run-tests.sh $@ && \
-docker-compose run php80 ./run-tests.sh $@ && \
-docker-compose run php81 ./run-tests.sh $@ && \
-docker-compose run php82 ./run-tests.sh $@ && \
-docker-compose run php83 ./run-tests.sh $@
+COMPOSE_CMD="mutagen-compose"
+
+$COMPOSE_CMD run --rm php71 ./run-tests.sh $@ && \
+$COMPOSE_CMD run --rm php72 ./run-tests.sh $@ && \
+$COMPOSE_CMD run --rm php73 ./run-tests.sh $@ && \
+$COMPOSE_CMD run --rm php74 ./run-tests.sh $@ && \
+$COMPOSE_CMD run --rm php80 ./run-tests.sh $@ && \
+$COMPOSE_CMD run --rm php81 ./run-tests.sh $@ && \
+$COMPOSE_CMD run --rm php82 ./run-tests.sh $@ && \
+$COMPOSE_CMD run --rm php83 ./run-tests.sh $@
diff --git a/tests/PHPUnit_Smarty.php b/tests/PHPUnit_Smarty.php
index de4a37769..bed598185 100644
--- a/tests/PHPUnit_Smarty.php
+++ b/tests/PHPUnit_Smarty.php
@@ -149,7 +149,7 @@ public function setUpSmarty($dir = null)
             $this->smarty->setCacheDir(__DIR__ . '/cache');
         }
 
-        $this->getSmartyObj();
+        $this->smarty->setErrorReporting(E_ALL &~ E_USER_DEPRECATED);
     }
 
     /**
diff --git a/tests/UnitTests/A_Core/LoadPlugin/LoadPluginTest.php b/tests/UnitTests/A_Core/LoadPlugin/LoadPluginTest.php
index 820f31a46..27f90a034 100644
--- a/tests/UnitTests/A_Core/LoadPlugin/LoadPluginTest.php
+++ b/tests/UnitTests/A_Core/LoadPlugin/LoadPluginTest.php
@@ -49,6 +49,6 @@ public function testLoadPluginSmartyTemplateClass()
      */
     public function testLoadPluginSmartyPluginCounter()
     {
-        $this->assertTrue($this->smarty->loadPlugin('Smarty_Function_Counter') == true);
+        $this->assertTrue($this->smarty->loadPlugin('Smarty_function_counter') == true);
     }
 }
diff --git a/tests/UnitTests/SecurityTests/SecurityTest.php b/tests/UnitTests/SecurityTests/SecurityTest.php
index 7631a0a6b..c3398f954 100644
--- a/tests/UnitTests/SecurityTests/SecurityTest.php
+++ b/tests/UnitTests/SecurityTests/SecurityTest.php
@@ -52,7 +52,7 @@ public function testSecurityLoaded()
  */
     public function testTrustedPHPFunction()
     {
-        $this->assertEquals("5", $this->smarty->fetch('string:{assign var=foo value=[1,2,3,4,5]}{sizeof($foo)}'));
+        $this->assertEquals("5", $this->smarty->fetch('string:{assign var=foo value=[1,2,3,4,5]}{count($foo)}'));
     }
 
 /**
@@ -75,7 +75,7 @@ public function testDisabledTrustedPHPFunction()
     {
         $this->smarty->security_policy->php_functions = array('null');
         $this->smarty->disableSecurity();
-        $this->assertEquals("5", $this->smarty->fetch('string:{assign var=foo value=[1,2,3,4,5]}{sizeof($foo)}'));
+        $this->assertEquals("5", $this->smarty->fetch('string:{assign var=foo value=[1,2,3,4,5]}{count($foo)}'));
     }
 
 /**
diff --git a/tests/UnitTests/SmartyMethodsTests/ClearAllAssign/ClearAllAssignTest.php b/tests/UnitTests/SmartyMethodsTests/ClearAllAssign/ClearAllAssignTest.php
index 9363f323f..5c1a679ff 100644
--- a/tests/UnitTests/SmartyMethodsTests/ClearAllAssign/ClearAllAssignTest.php
+++ b/tests/UnitTests/SmartyMethodsTests/ClearAllAssign/ClearAllAssignTest.php
@@ -46,7 +46,7 @@ public function testAllVariablesAccessable()
      */
     public function testClearAllAssignInTemplate()
     {
-        error_reporting((error_reporting() & ~(E_NOTICE | E_USER_NOTICE | E_WARNING)));
+        $this->smarty->setErrorReporting($this->smarty->error_reporting & ~(E_NOTICE | E_USER_NOTICE | E_WARNING));
         $this->_tpl->clearAllAssign();
         $this->assertEquals('foobar', $this->smarty->fetch($this->_tpl));
     }
@@ -56,7 +56,7 @@ public function testClearAllAssignInTemplate()
      */
     public function testClearAllAssignInData()
     {
-        error_reporting((error_reporting() & ~(E_NOTICE | E_USER_NOTICE | E_WARNING)));
+        $this->smarty->setErrorReporting($this->smarty->error_reporting & ~(E_NOTICE | E_USER_NOTICE | E_WARNING));
         $this->_data->clearAllAssign();
         $this->assertEquals('fooblar', $this->smarty->fetch($this->_tpl));
     }
@@ -66,7 +66,7 @@ public function testClearAllAssignInData()
      */
     public function testClearAllAssignInSmarty()
     {
-        error_reporting((error_reporting() & ~(E_NOTICE | E_USER_NOTICE | E_WARNING)));
+        $this->smarty->setErrorReporting($this->smarty->error_reporting & ~(E_NOTICE | E_USER_NOTICE | E_WARNING));
         $this->smarty->clearAllAssign();
         $this->assertEquals('barblar', $this->smarty->fetch($this->_tpl));
     }
diff --git a/tests/UnitTests/TemplateSource/ValueTests/PHPfunctions/PhpFunctionTest.php b/tests/UnitTests/TemplateSource/ValueTests/PHPfunctions/PhpFunctionTest.php
index cf8970c5d..9a3f63191 100644
--- a/tests/UnitTests/TemplateSource/ValueTests/PHPfunctions/PhpFunctionTest.php
+++ b/tests/UnitTests/TemplateSource/ValueTests/PHPfunctions/PhpFunctionTest.php
@@ -57,6 +57,7 @@ public function testEmpty1()
     public function testEmpty2()
     {
         $this->smarty->disableSecurity();
+	    $this->smarty->registerPlugin('modifier', 'pass', 'pass');
         $this->smarty->assign('var', array(null,
                                            false,
                                            (int) 0,
@@ -78,6 +79,7 @@ public function testEmpty2()
     public function testEmpty3()
     {
         $this->smarty->disableSecurity();
+	    $this->smarty->registerPlugin('modifier', 'pass', 'pass');
         $this->smarty->assign('var', array(true,
                                            (int) 1,
                                            (float) 0.1,
@@ -114,6 +116,7 @@ public function testEmpty4()
     public function testIsset1()
     {
         $this->smarty->disableSecurity();
+	    $this->smarty->registerPlugin('modifier', 'pass', 'pass');
         $this->smarty->assign('isNull', null);
         $this->smarty->assign('isSet', 1);
         $this->smarty->assign('arr', array('isNull' => null, 'isSet' => 1));
@@ -155,7 +158,7 @@ public function testIsset2()
     public function testIsset3($strTemplate, $result)
     {
         $this->smarty->disableSecurity();
-
+	    $this->smarty->registerPlugin('modifier', 'intval', 'intval');
         $this->smarty->assign('varobject', new TestIsset());
         $this->smarty->assign('vararray', $vararray = array(
             'keythatexists' => false,
@@ -196,6 +199,105 @@ public function dataTestIsset3()
             array('{if isset($_varsimple{$key})}true{else}false{/if}', 'true'),
         );
     }
+
+	/**
+	 * Tests various PHP functions (deprecated)
+	 * @dataProvider        dataVariousPHPFunctions
+	 */
+	public function testVariousPHPFunctions($strTemplate, $value, $expected) {
+		$this->smarty->disableSecurity();
+		$this->cleanDirs();
+		$this->smarty->assign('value', $value);
+		$this->assertEquals($expected, $this->smarty->fetch('string:' . $strTemplate));
+	}
+
+	/**
+	 * Data provider for testIsset3
+	 */
+	public function dataVariousPHPFunctions()
+	{
+		return array(
+			array('{$a = count($value)}{$a}', array(1,2,3), '3'),
+			array('{$a = in_array("b", $value)}{$a}', array(1,'b',3), true),
+			array('{$a = strlen(uniqid())}{$a}', '', 13),
+			array('{$a = date("Y", $value)}{$a}', strtotime("01-01-2030"), 2030),
+			array('{$a = PhpFunctionTest::sayHi($value)}{$a}', 'mario', 'hi mario'),
+			array('{$a = pass($value)}{$a}', 'mario', 'mario'),
+			array('{$a = 1}{$b = Closure::fromCallable($value)}{$a}', 'strlen', 1),
+		);
+	}
+
+	public static function sayHi($value) {
+		return 'hi ' . $value;
+	}
+
+	/**
+	 * Tests that each function that will not be supported in Smarty 5 does throw an E_USER_DEPRECATED notice.
+	 * @dataProvider        dataDeprecationNoticesForSmarty5
+	 * @deprecated
+	 */
+	public function testDeprecationNoticesForSmarty5($strTemplateSource, $expected = '', $shouldTriggerDeprecationNotice = false) {
+
+		// this overrides the error reporting level set in \PHPUnit_Smarty::setUpSmarty
+		$previousErrorReporting = $this->smarty->error_reporting;
+		$this->smarty->setErrorReporting($previousErrorReporting | E_USER_DEPRECATED);
+
+		$this->smarty->assign('a', 'a');
+		$this->smarty->assign('ar', [1,2]);
+		$this->smarty->assign('f', 3.14);
+
+		$errorMessage = '';
+
+		try {
+			$output = $this->smarty->fetch('string:' . $strTemplateSource);
+		} catch (Exception $e) {
+			$errorMessage = $e->getMessage();
+		}
+
+		if ($shouldTriggerDeprecationNotice) {
+			$this->assertStringContainsString('Using unregistered function', $errorMessage);
+		} else {
+			$this->assertEquals($expected, $output);
+			$this->assertEquals('', $errorMessage);
+		}
+
+		$this->smarty->setErrorReporting($previousErrorReporting);
+	}
+
+	public function dataDeprecationNoticesForSmarty5()
+	{
+
+		return [
+
+			['{if empty($a)}{else}b{/if}', 'b', false],
+			['{json_encode($a)}', '"a"', false],
+			['{nl2br($a)}', 'a', false],
+			['{$a|nl2br}', 'a', false],
+			['{round($f, 1)}', '3.1', false],
+			['{$f|round}', '3', false],
+			['{str_repeat($a, 2)}', 'aa', false],
+			['{$a|str_repeat:3}', 'aaa', false],
+			['{$a|strip_tags}', 'a', false],
+			['{$a|strlen}', '1', false],
+			['{$a|substr:-1}', 'a', false],
+			['{$f|substr:-1}', '4', false],
+			['{$ar|count}', '2', false],
+			['{foreach "."|explode:$f as $n}{$n}{/foreach}', '314', false],
+			['{"-"|implode:$ar}', '1-2', false],
+			['{"-"|join:$ar}', '1-2', false],
+			['{$f|wordwrap:2:"k":true}', "3.k14", false],
+			['{$f|number_format:1:","}', "3,1", false],
+			['{if in_array(1, $ar)}yes{/if}', "yes", false],
+			['{if is_array($ar)}yes{/if}', "yes", false],
+			['{if time() gt 0}yes{/if}', "yes", false],
+
+			['{if array_chunk($ar, 2)}x{else}y{/if}', '', true],
+			['{$a|addslashes}', '', true],
+			['{$a|sha1}', '', true],
+			['{$a|get_parent_class}', '', true],
+		];
+	}
+
 }
 
 /**