<meta charset="utf-8">
<?php

$arr = ['a' => 1, 'b' => 2, 'c' => 3, 'd' => 4,'e'=>5];
function foo($var){ 
     return($var & 1); 
}
echo "<pre>";
print_r(array_filter($arr,function($var){
    return($var & 1);   //二進位制的 & ^ | << >> 運算.：先把1轉換成00000001在於變數$var進行與運算，奇數結果為真，偶數結果為假
}));//Array([a] => 1    [c] => 3    [e] => 5)