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run code in 300+ PHP versions simultaneously
<?php spl_autoload_register(function($class) { if ($class === 'MyClass') { eval(<<<'PHP' class MyClass {} PHP ); } }); register_shutdown_function(function() { new MyClass; }); eval(<<<'PHP' $x = ['x']; foreach ($x as $i) { $y[]; } PHP );

preferences:
28.54 ms | 406 KiB | 5 Q