请教这个php程式为什么会出错

请问这个php程式为什么会出错。

PHP code

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

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

---------------------------------------------
Warning: Wrong parameter count for ucwords() in E:\wwwroot\webtest\test.php on line 25

------解决方案--------------------

PHP code

$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('', $arr2);
<br><font color="#e78608">------解决方案--------------------</font><br><br><br>$array = explode('_', $string);<br><br>array_walk($array, "ucwords");<br><br><br>$array前加多个$array=array(); <div class="clear">
                 
              
              
        
            </div>