Better way to check variable for null or empty string?

if (strlen($str) == 0){
    // do whatever
}

Replace $str with your variable. NULL and "" both return 0 when using strlen.

Since PHP 8.1 passing NULL to strlen() will cause a depreciation notice. https://3v4l.org/0s1Gs If you know the variable type to be either string or null then you can check it as so:

if ($str === null || strlen($str) == 0){
    // do whatever
}

Use PHP's empty() function. The following things are considered to be empty

"" (an empty string)
0 (0 as an integer)
0.0 (0 as a float)
"0" (0 as a string)
NULL
FALSE
array() (an empty array)
$var; (a variable declared, but without a value)

For more details check empty function

I'll humbly accept if I'm wrong, but I tested on my own end and found that the following works for testing both string(0) "" and NULL valued variables:

if ( $question ) {
  // Handle success here
}

Which could also be reversed to test for success as such:

if ( !$question ) {
  // Handle error here
}

I'd recommend using the null coalescing operator. This will also supress an error in the case that the array key does not exist.

if (trim($_POST['question'] ?? '') === '') {
    // Handle error here
}

Beware false negatives from the trim() function — it performs a cast-to-string before trimming, and thus will return e.g. "Array" if you pass it an empty array. That may not be an issue, depending on how you process your data, but with the code you supply, a field named question[] could be supplied in the POST data and appear to be a non-empty string. Instead, I would suggest:

$question = $_POST['question'];

if (!is_string($question) || ($question = trim($question))) {
    // Handle error here
}

// If $question was a string, it will have been trimmed by this point

There is no better way but since it's an operation you usually do quite often, you'd better automatize the process.

Most frameworks offer a way to make arguments parsing an easy task. You can build you own object for that. Quick and dirty example :

class Request
{

    // This is the spirit but you may want to make that cleaner :-)
    function get($key, $default=null, $from=null)
    {
         if ($from) :
             if (isset(${'_'.$from}[$key]));
                return sanitize(${'_'.strtoupper($from)}[$key]); // didn't test that but it should work
         else
             if isset($_REQUEST[$key])
                return sanitize($_REQUEST[$key]);

         return $default;
    }

    // basics. Enforce it with filters according to your needs
    function sanitize($data)
    {
          return addslashes(trim($data));
    }

    // your rules here
    function isEmptyString($data)
    {
        return (trim($data) === "" or $data === null);
    }


    function exists($key) {}

    function setFlash($name, $value) {}

    [...]

}

$request = new Request();
$question= $request->get('question', '', 'post');
print $request->isEmptyString($question);

Symfony use that kind of sugar massively.

But you are talking about more than that, with your "// Handle error here ". You are mixing 2 jobs : getting the data and processing it. This is not the same at all.

There are other mechanisms you can use to validate data. Again, frameworks can show you best pratices.

Create objects that represent the data of your form, then attach processses and fall back to it. It sounds far more work that hacking a quick PHP script (and it is the first time), but it's reusable, flexible, and much less error prone since form validation with usual PHP tends to quickly become spaguetti code.

This one checks arrays and strings:

function is_set($val) {
  if(is_array($val)) return !empty($val);

  return strlen(trim($val)) ? true : false;
}

When you want to check if a value is provided for a field, that field may be a string , an array, or undifined. So, the following is enough

function isSet($param)
{
    return (is_array($param) && count($param)) || trim($param) !== '';
}