How to make an associative array from values of two normal arrays?

Question

CODE

<?php
    $a = array('31','01','02','03','30','31','01','02');
    $b = array('7','7','7','7','8','8','8','8');
    $c = array_combine($a,$b);
    var_dump($c);
?>

RESULT It ignores the duplicate values with the same key array(5) { [31]=> string(1) "8" ["01"]=> string(1) "8" ["02"]=> string(1) "8" ["03"]=> string(1) "7" [30]=> string(1) "8" }

WHAT I NEED

I need to make an associative array with keys and duplicate values separated by comma.(Here: 31th key has two values 7 and 8)

Answer 1

Your task can be solved by a simple foreach:

$c = [];
foreach ($a as $key => $value) {
    if (!isset($c[$value])) {
        // there's NO key `$value` in `$c` - add it with initial value
        $c[$value] = [];
    } 

    // add new value to this key
    $c[$value][] = $b[$key];
}
// Output
// Array ( [31] => Array ( [0] => 7 [1] => 8 ) [01] => Array ( [0] => 7 [1] => 8 ) [02] => Array ( [0] => 7 [1] => 8 ) [03] => Array ( [0] => 7 ) [30] => Array ( [0] => 8 ) 

Version for comma separated values:

foreach ($a as $key => $value) {
    if (!isset($c[$value])) {
        // there's NO key `$value` in `$c` - add it with initial value
        $c[$value] = $b[$key];
    } else {
        $c[$value] .= ',' . $b[$key];
    }
}
// Output:
// Array ( [31] => 7,8 [01] => 7,8 [02] => 7,8 [03] => 7 [30] => 8 )