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run code in 200+ php & hhvm versions
Bugs & Features
<?php $test = array('key' => 'value'); print_r( array_reduce( array_keys( array_filter( $test, function($key,$value){ return $value == 'value';}, ARRAY_FILTER_USE_BOTH ) ), function($carry,$value){return $carry . $value;} ) );
based on OIUJG
Output for 5.6.30, hhvm-3.18.5 - 3.22.0, 7.0.30 - 7.3.0beta1