如何将数组转换成键值对字符串

function array2string($array){
 
    $string = [];
 
    if($array && is_array($array)){
 
        foreach ($array as $key=> $value){
            $string[] = $key.'->'.$value;
        }
    }
 
    return implode(',',$string);
}
$array1= array('a', 'b', 'c', 'd');
echo $a = array2string($array1);