- var_dump: documentation ( source)
- array_column: documentation ( source)
<?php
$a = array(
array('id' => 3, 'a' => 33),
array('id' => 5, 'a' => 55),
array('id' => 7, 'b' => 256),
array('id' => '', 'a'=> 512),
);
var_dump(array_column($a, null, 'id'));