<?php
 
$a = array(
     0 => 9,
     2 => 13
     );
 
$b = array(
     1 => 10,
     2 => 11,
     3 => 12,
     4 => 1
     );
 
 $ab = mergeArrays($a, $b);
 print_r($ab);
 
 function mergeArrays($a, $b) {
    $adjust = 0;
    foreach ($b as $i => $val) {
        while (isset($a[$i + $adjust])) {
            $adjust++;
        }
        $a[$i + $adjust] = $val;
    }
    ksort($a); // Put back in order by indexes
    return $a;
}

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