PHP: if (!$val) VS if (empty($val)). Is there any difference?

Have a look at the PHP type comparison table.

If you check the table, you'll notice that for all cases, empty($x) is the same as !$x. So it comes down to handling uninitialised variables. !$x creates an E_NOTICE, whereas empty($x) does not.

If you use empty and the variable was never set/created, no warning/error will be thrown.

Let see:

empty documentation:

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 $var; (a variable declared, but without a value in a class)

Booleans documentation:

When converting to boolean, the following values are considered FALSE:

• the boolean FALSE itself
• the integer 0 (zero)
• the float 0.0 (zero)
• the empty string, and the string "0"
• an array with zero elements
• an object with zero member variables (PHP 4 only)
• the special type NULL (including unset variables)
• SimpleXML objects created from empty tags

It seems the only difference (regarding the resulting value) is how a SimpleXML instance is handled. Everything else seems to give the same result (if you invert the boolean cast of course).