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run code in 150+ php & hhvm versions
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<?php function foo($arg){ var_dump($arg); } preg_replace('/a/', foo("$1"), 'a');
Output for 4.3.0 - 5.6.28, hhvm-3.10.0 - 3.12.0, 7.0.0 - 7.1.0
string(2) "$1"