PHP best way to check whether a string is empty or not

You need isset() in case $str is possibly undefined:

if (isset($str) && $str !== '') {
    // variable set, not empty string
}

Using !empty() would have an important caveat: the string '0' evaluates to false.

Also, sometimes one wants to check, in addition, that $str is not something falsy, like false or null[1]. The previous code doesn't handle this. It's one of the rare situations where loose comparison may be useful:

if (isset($str) && $str != '') {
    // variable set, not empty string, not falsy
}

The above method is interesting as it remains concise and doesn't filter out '0'. But make sure to document your code if you use it.

Otherwise you can use this equivalent but more verbose version:

if (isset($str) && (string) $str !== '') {
    // variable set, not empty string, not falsy
}

Of course, if you are sure $str is defined, you can omit the isset($str) from the above codes.

Finally, considering that '' == false, '0' == false, but '' != '0', you may have guessed it: PHP comparisons aren't transitive (fun graphs included).

[1] Note that isset() already filters out null.

Since PHP will treat a string containing a zero ('0') as empty, it makes the empty() function an unsuitable solution.

Instead, test that the variable is explicitly not equal to an empty string:

$stringvar !== ''

As the OP and Gras Double and others have shown, the variable should also be checked for initialization to avoid a warning or error (depending on settings):

isset($stringvar)

This results in the more acceptable:

if (isset($stringvar) && $stringvar !== '') {
}

PHP has a lot of bad conventions. I originally answered this (over 9 years ago) using the empty() function, as seen below. I've long since abandoned PHP, but since this answer attracts downvotes and comments every few years, I've updated it. Should the OP wish to change the accepted answer, please do so.

Original Answer:

if(empty($stringvar))
{
    // do something
}

You could also add trim() to eliminate whitespace if that is to be considered.

Edit:

Note that for a string like '0', this will return true, while strlen() will not.