function all_combinations(&$letters, &$combinations) {
for($i = 0; $i < count($letters); $i++) {
$combinations[] = $letters[$i];
$partlet = $letters;
array_splice($partlet, $i, 1);
$partcomb = array();
all_combinations($partlet, $partcomb);
foreach($partcomb as $comb) {
$combinations[] = $letters[$i] . $comb;
}
}
}
$combinations = array();
$letters = array("a", "b", "c", "d");
all_combinations($letters, $combinations);
echo "<pre>";
var_dump($combinations);
echo "</pre>";
/*
run:
array(64) {
[0]=>
string(1) "a"
[1]=>
string(2) "ab"
[2]=>
string(3) "abc"
[3]=>
string(4) "abcd"
[4]=>
string(3) "abd"
[5]=>
string(4) "abdc"
[6]=>
string(2) "ac"
[7]=>
string(3) "acb"
[8]=>
string(4) "acbd"
[9]=>
string(3) "acd"
[10]=>
string(4) "acdb"
[11]=>
string(2) "ad"
[12]=>
string(3) "adb"
[13]=>
string(4) "adbc"
[14]=>
string(3) "adc"
[15]=>
string(4) "adcb"
[16]=>
string(1) "b"
[17]=>
string(2) "ba"
[18]=>
string(3) "bac"
[19]=>
string(4) "bacd"
[20]=>
string(3) "bad"
[21]=>
string(4) "badc"
[22]=>
string(2) "bc"
[23]=>
string(3) "bca"
[24]=>
string(4) "bcad"
[25]=>
string(3) "bcd"
[26]=>
string(4) "bcda"
[27]=>
string(2) "bd"
[28]=>
string(3) "bda"
[29]=>
string(4) "bdac"
[30]=>
string(3) "bdc"
[31]=>
string(4) "bdca"
[32]=>
string(1) "c"
[33]=>
string(2) "ca"
[34]=>
string(3) "cab"
[35]=>
string(4) "cabd"
[36]=>
string(3) "cad"
[37]=>
string(4) "cadb"
[38]=>
string(2) "cb"
[39]=>
string(3) "cba"
[40]=>
string(4) "cbad"
[41]=>
string(3) "cbd"
[42]=>
string(4) "cbda"
[43]=>
string(2) "cd"
[44]=>
string(3) "cda"
[45]=>
string(4) "cdab"
[46]=>
string(3) "cdb"
[47]=>
string(4) "cdba"
[48]=>
string(1) "d"
[49]=>
string(2) "da"
[50]=>
string(3) "dab"
[51]=>
string(4) "dabc"
[52]=>
string(3) "dac"
[53]=>
string(4) "dacb"
[54]=>
string(2) "db"
[55]=>
string(3) "dba"
[56]=>
string(4) "dbac"
[57]=>
string(3) "dbc"
[58]=>
string(4) "dbca"
[59]=>
string(2) "dc"
[60]=>
string(3) "dca"
[61]=>
string(4) "dcab"
[62]=>
string(3) "dcb"
[63]=>
string(4) "dcba"
}
*/
