- array_splice: documentation ( source)
- print_r: documentation ( source)
- ceil: documentation ( source)
<?php
$arr1 = array('1', '2', '3', '4', '5', '6', '7');
$arr2 = array('a', 'b', 'c', 'd', 'e');
$count = ceil(count($arr1) / 3) - 1;
for ($i = $count - 1; $i >= 0; $i--) {
array_splice($arr1, ($i + 1) * 3, 0, $arr2[$i]);
}
print_r($arr1);