merge_two_arrays.php

<?php

$array1 = array(1, 2, 3, 4, 5);
$array2 = array(6,7,8,9);

$arrays_total_length = count($array1) + count($array2);

$index1 = 0;
$index2 = 0;
$output_array = array();

for ($i = 0; $i < $arrays_total_length; $i++) {
	if ($i % 2 == 0) {
		array_push($output_array, $array1[$index1]);
		$index1++;
	}

	if ($i % 2 != 0) {
		array_push($output_array, $array2[$index2]);
		$index2++;
	}
}

print_r($output_array);