Split the illegal string offsets example output by PHP version (#5232)

The output shown for the example was already corrected in master; what the
issue thread still asked for was making the version difference visible.

Split the output into two sections with example.outputs.8 and
example.outputs.7, and state the PHP 8 rule in the introductory paragraph.

State the PHP 8 rule as it actually is. A string starting with an integer
followed by other characters, such as "1x", does not throw a TypeError: it
still emits E_WARNING and is read as its leading integer, which the PHP 8
output of this very example shows. Only the other non-integer-like strings
throw.

The PHP 7 output is what PHP 7 really prints: isset() rejects every offset
that is not integer-like, so it is false for both '1.0' and '1x', and '1x'
emits "A non well formed numeric value encountered" rather than an illegal
offset warning, since the read path allows trailing data.

Sources:
- Zend/zend_execute.c, zend_isset_dim_slow() and the string branch of
  zend_fetch_dimension_address_read(); Zend/zend_operators.c,
  _is_numeric_string_ex()
- Zend/tests/offset_string.phpt and Zend/tests/isset_str_offset.phpt in 7.4
- PHP 8 output verified against PHP 8.5.4

Author: lacatoire

language/types/string.xml

@@ -1088,10 +1088,15 @@ var_dump($str);
     </programlisting>
    </example>
 
-   <para>
-    String offsets have to either be integers or integer-like strings,
-    otherwise a warning will be thrown.
-   </para>
+   <simpara>
+    String offsets must be either integers or integer-like strings.
+    As of PHP 8.0.0, any other string throws a
+    <exceptionname>TypeError</exceptionname>, except for a string starting
+    with an integer followed by other characters, which emits
+    <constant>E_WARNING</constant> and is interpreted as that leading integer.
+    Prior to PHP 8.0.0, no <exceptionname>TypeError</exceptionname> was
+    thrown: an illegal offset emitted <constant>E_WARNING</constant> instead.
+   </simpara>
 
    <example>
     <title>Example of Illegal String Offsets</title>
@@ -1116,7 +1121,7 @@ foreach ($keys as $keyToTry) {
 ?>
 ]]>
     </programlisting>
-    &example.outputs;
+    &example.outputs.8;
     <screen>
 <![CDATA[
 bool(true)
@@ -1130,7 +1135,29 @@ Cannot access offset of type string on string
 
 bool(false)
 
-Warning: Illegal string offset "1x" in Standard input code on line 10
+Warning: Illegal string offset "1x" in example.php on line 10
+string(1) "b"
+]]>
+    </screen>
+    &example.outputs.7;
+    <screen>
+<![CDATA[
+bool(true)
+string(1) "b"
+
+bool(false)
+
+Warning: Illegal string offset '1.0' in example.php on line 10
+string(1) "b"
+
+bool(false)
+
+Warning: Illegal string offset 'x' in example.php on line 10
+string(1) "a"
+
+bool(false)
+
+Notice: A non well formed numeric value encountered in example.php on line 10
 string(1) "b"
 ]]>
     </screen>