如何将变量作为键？

How can I make a variable into a key or a value so that I can use it in in_array()?

For instance,

// set the variable
$cnt_firstname = trim($_POST['cnt_firstname']);
$required = trim($_POST['required']);

// set the array for required fields
$array_required = explode(' ',$required);

// check the item is not empty and if it is in the array of required
if(empty($cnt_firstname) && in_array($cnt_firstname, $array_required))
{
$error = true;
echo '<error elementid="cnt_firstname" message="Please enter your fist name." />';
}

this is the post data I would send from a form,

Array
(
    [cnt_firstname] =>
    [cnt_lastname] => 
    [cnt_organisation] => 
    [cnt_email1] => 
    [cnt_telephpne] => 
    [required] => cnt_firstname cnt_lastname cnt_email1
)

I cannot get the result. I want it to return false if the field is empty and that field is in required array.

How can I make it work?

Quick & Dirty:

if(empty($cnt_firstname) && in_array('cnt_firstname', $array_required))

The way you wrote it won't work, because it will only check, if $cnt_firstname is in the required array, if it's empty - in other words: is an empty string in array?