PrefixAllGlobals: add "php" to the list of blacklisted prefixes

The `php` prefix is reserved for PHP itself.

No unit tests added as the logic for this blacklist is already sufficiently unit tested.

Fixes 1728

Author: jrfnl

WordPress/Sniffs/NamingConventions/PrefixAllGlobalsSniff.php

@@ -65,6 +65,7 @@ class PrefixAllGlobalsSniff extends AbstractFunctionParameterSniff {
 		'wordpress' => true,
 		'wp'        => true,
 		'_'         => true,
+		'php'       => true, // See #1728, the 'php' prefix is reserved by PHP itself.
 	);
 
 	/**