<?php

$array = [
    ['b', 'e', 'd', 's'],
    ['l', 'n']
];

$maxCount = max(array_map('count', $array));
$result = [];
for ($i = 0; $i < $maxCount; ++$i) {
	foreach (array_column($array, $i) as $found) {
		$result[] = $found;
	}
}
var_export($result);