- print_r: documentation ( source)
<?php
$arr1 = array('1', '2', '3', '4', '5', '6', '7');
$arr2 = array('a', 'b', 'c', 'd', 'e');
$result = [];
foreach($arr1 as $key => $value){
$result[] = $value;
if((0 == ($key + 1) % 3) && isset($arr2[($key + 1)/3-1])){
$result[] = $arr2[($key + 1)/3-1];
}
}
print_r($result);