If an entry in the source array does not have a column_key element then array_column will silently skip that entry and return an array shorter than the source.
If entries can't be uniquely identified by an index_key then there's no way of telling which ones were skipped, as without an index_key array_column returns a plain list.
<?php
$array = [
['a' => '0th', 'b' => 'zero'],
['a' => '1st', 'b' => 'one'],
['a' => '2nd' ],
['a' => '3rd', 'b'=>'three']];
var_export(array_column($array, 'b'));
var_export(array_column($array, 'b', 'a'));
?>
