Drop support for PHPUnit < 6.4 [1]

Adjust CI, README and `composer.json` for the drop of support for PHPUnit < 6.4.4.

The 6.4.4 version is a deliberate choice to allow for dropping:
* The `ExpectExceptionObject` polyfill (native method was introduced in PHPUnit 6.4.0 with bugfix in PHPUnit 6.4.4)
* A work-around in the `AssertStringContains` polyfills for a bug which was fixed in PHPUnit 6.4.2.
* And not requiring a call to `expectException()` for a call to `expectExceptionMessage()` (bug fixed in PHPUnit 6.4.3).

.github/workflows/test.yml

@@ -29,20 +29,21 @@ jobs:
         include:
           # Test against a version on the low-end of the PHPUnit versions supported for each PHP version.
           # Using the Composer `--prefer-lowest` option is, unfortunately, not viable, as
-          # it would result PHP 7.0 - 7.4 all using PHPUnit 5.7.21, which is not the intention.
+          # it would result PHP 7.0 - 7.4 all using PHPUnit 6.4.4, which is not the intention.
           # It also would run into trouble with PHP 8.5.12 being used on PHP 8.0+, while the
           # 8.5.12 release still contained a bug which makes it incompatible with PHP 8.1+,
           # even though it officially allows for it..
           - php: '7.0'
-            phpunit: '5.7.27'
+            phpunit: '6.4.4'
             coverage: true
             experimental: false
           - php: '7.1'
-            phpunit: '5.7.21'
+            phpunit: '~6.4.4'
             coverage: true
             experimental: false
           - php: '7.2'
-            phpunit: '6.3.1'
+            # - PHPUnit 7.0 seems to have an issue with something related to TestListeners, so using PHPUnit 7.1 instead for "low".
+            phpunit: '~7.1.0'
             coverage: true
             experimental: false
           - php: '7.3'
@@ -199,24 +200,13 @@ jobs:
           # and very select other combinations.
           # This should be sufficient to record the coverage for the PHAR specific code.
 
-          # PHPUnit 5 is only supported for PHPUnit 5.7.21-latest.
+          # PHPUnit 6 is only supported for PHPUnit 6.4.4-latest on the officially supported PHP versions.
           - php: '7.0'
-            phpunit: '5.7.21'
-          - php: '7.0'
-            phpunit: '5'
-          - php: '7.1'
-            phpunit: '5.7.21'
-          - php: '7.1'
-            phpunit: '5'
-            coverage: true
-
-          # PHPUnit 6 is fully supported for the officially supported PHP versions.
-          - php: '7.0'
-            phpunit: '6.0'
+            phpunit: '6.4'
           - php: '7.0'
             phpunit: '6'
           - php: '7.2'
-            phpunit: '6.0'
+            phpunit: '6.4'
           - php: '7.2'
             phpunit: '6'
             coverage: true

README.md

@@ -35,7 +35,7 @@ Requirements
 ------------
 
 * PHP 7.0 or higher.
-* [PHPUnit] 5.7 - 10.x (automatically required via Composer).
+* [PHPUnit] 6.4 - 10.x (automatically required via Composer).
 
 [PHPUnit]: https://packagist.org/packages/phpunit/phpunit
 
@@ -65,7 +65,7 @@ Why use the PHPUnit Polyfills?
 
 This library is set up to allow for creating PHPUnit cross-version compatible tests by offering a number of polyfills for functionality which was introduced, split up or renamed in PHPUnit.
 
-### Write your tests for PHPUnit 10.x and run them on PHPUnit 5.7 - 10.x
+### Write your tests for PHPUnit 10.x and run them on PHPUnit 6.4 - 10.x
 
 The polyfills have been setup to allow tests to be _forward_-compatible. What that means is, that your tests can use the assertions supported by the _latest_ PHPUnit version, even when running on older PHPUnit versions.
 
@@ -163,11 +163,11 @@ You will need to make sure to `use` any additional traits needed for the polyfil
 
 | PHPUnit   | When `use`-ing this trait   | You also need to `use` this trait |
 | --------- | --------------------------- | --------------------------------- |
-| 5.7 < 7.5 | `AssertIgnoringLineEndings` | `AssertStringContains`            |
+| 6.4 < 7.5 | `AssertIgnoringLineEndings` | `AssertStringContains`            |
 
 _**Note: this only applies to the stand-alone use of the traits. The [`TestCase` classes](#testcases) provided by this library already take care of this automatically.**_
 
-Code example for a test using the `AssertIgnoringLineEndings` trait, which needs to be able to run on PHPUnit 5.7:
+Code example for a test using the `AssertIgnoringLineEndings` trait, which needs to be able to run on PHPUnit 6.4:
 ```php
 <?php
 

composer.json

@@ -27,7 +27,7 @@
 	},
 	"require": {
 		"php": ">=7.0",
-		"phpunit/phpunit": "^5.7.21 || ^6.0 || ^7.0 || ^8.0 || ^9.0 || ^10.0"
+		"phpunit/phpunit": "^6.4.4 || ^7.0 || ^8.0 || ^9.0 || ^10.0"
 	},
 	"require-dev": {
 		"php-parallel-lint/php-console-highlighter": "^1.0.0",