- array_column: documentation ( source)
- print_r: documentation ( source)
<?php
$array = array(
0 => array( 'key' => 'val1' ),
1 => array( 'key' => 'val2' ),
2 => array( 'key' => 'val3' ),
);
$new_array = array_column($array, 'key');
print_r($new_array);