请教这个php程式为什么会出错

$s2 = "make_by_id";
echo convString($s1);

function convString($string)
{
    $array = explode('_', $string);
    array_walk($array, "ucwords");
    return implode('', $array);
}

$arr2    = explode('_', $s2);
function self_ucwords(&$p_a, $p_b)//通常传入2参数，也可以3个参数，更详细用法查手册
{
    $p_a    = ucwords($p_a);//一个参数
}
array_walk($arr2, 'self_ucwords');
echo    implode('', $arr2);

------解决方案--------------------


$array = explode('_', $string);

array_walk($array, "ucwords");


$array前加多个$array=array();