- array_slice: documentation ( source)
- print_r: documentation ( source)
- array_merge: documentation ( source)
<?php
$a = array( 'a','b','c','d','e','f','d' );
$b = array( 'x' );
$arr = array_merge(array_slice($a, 0, 3), $b, array_slice($a, 3));
print_r($arr);