<?php
function odd($var) {
    // returns whether the input integer is odd
    return($var & 1);
 }

function calculate() {

    $array1 = array("a"=>1, "b"=>2, "c"=>3, "d"=>4, "e"=>5);
    $array2 = array(6, 7, 8, 9, 10, 11, 12);

    echo "Odd :\n";
    print_r(array_filter($array1, "odd"));
}

calculate();

Odd :
Array
(
    [a] => 1
    [c] => 3
    [e] => 5
)