- var_dump: documentation ( source)
- array_intersect: documentation ( source)
- array_keys: documentation ( source)
<?php
$a1=array("a"=>"red","b"=>"green","c"=>"blue","d"=>"yellow");
$a2=array("e"=>"red","f"=>"green","g"=>"blue");
var_dump(array_intersect(array_keys($a1), array_keys($a2)));