- array_push: documentation ( source)
- array_splice: documentation ( source)
- var_export: documentation ( source)
<?php
$arr1 = [1, 2, 3, 7, 8, 9];
$arr2 = [4, 5, 6, 10, 11, 12, 13, 14];
$result = [];
while ($arr1 || $arr2) {
array_push($result, ...array_splice($arr1, 0, 3), ...array_splice($arr2, 0, 3));
}
var_export($result);