- var_dump: documentation ( source)
- array_intersect_key: documentation ( source)
- array_flip: documentation ( source)
<?php
$formats = array(
'plain_text' => 'plain_text',
);
$elements['#allowed_formats'] = array();
$formats = array_intersect_key($formats, array_flip($elements['#allowed_formats']));
var_dump($formats);