<?php
 
function combine($inputArray, &$outputArray, $index, $combLen) {
        global $outstr;
        for ($i = $index; $i < count($inputArray); $i++) {
                $outstr.=$inputArray[$i];
                if(strlen($outstr) == $combLen) {
                        $outputArray[]= $outstr;
                }
                combine($inputArray, $outputArray, $i + 1, $combLen);
                $outstr = substr($outstr, 0, strlen($outstr)-1);
        }
}
 
$arr = array('A','B','C','D');
for($i=1; $i<=4; $i++) {
        echo "Combinations of length $i\n";
        $out = array();
        combine($arr, $out, 0, $i);
        var_dump($out);
}
 

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