php – get characters that come after a certain symbol has occured twice

eg. this string:

$foo = 'this_is_a_string';

Now I want to extract the string that comes after the second _ symbol, no matter how long the string is or how many _ symbols it contains.

this should be the result:

"a_string"

>Solution :

<?php
$foo = 'this_is_a_string';
var_dump(substr($foo,stripos($foo,'_',stripos($foo,'_')+1)+1));
?>