php - Why is if(empty(strlen(trim($_POST['name'])))) invalid?

I have a if statement check to see if a string is empty

if(empty(strlen(trim($_POST['name'])))){
    $error_empty = true;
}

gives me this error:

Fatal error: Can't use function return value in write context in C:\xampp\htdocs\requestaccess\index.php on line 51

Answer

empty is not a function -- it's a "language construct" that prior to PHP 5.5 can only be used to evaluate variables, not the results of arbitrary expressions.

If you wanted to use empty in exactly this manner (which is meaningless) you would have to store the result in an intermediate variable:

$var = strlen(trim($_POST['name']));
if(empty($var)) ...

But you don't need to do anything like this: strlen will always return an integer, so empty will effectively be reduced to checking for zero. You don't need empty for that; zero converts to boolean false automatically, so a saner option is

if(!strlen(trim($_POST['name']))) ...

or the equivalent

if(trim($_POST['name']) === '') ...