Converting php array of arrays into single array

why complicate things?

foreach($array as $k=>$v) {
    $new[$k] = $v['username'];
}

Since PHP 5.6 you can use the splat operator. In case array keys are numeric, you could use this:

$newArray = array_merge(...$oldArray);

Since PHP 5.5.0 there is a built-in function array_column which does exactly this.

$new_array = array_column($last_array, 'username');