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问题:

I was wondering what's the difference the two cases below, and which one is recommended?

$val = 0;

if (!$val) {
  //True
}

if (empty($val) {
  //It's also True
}

回答1:

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.

回答2:

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

回答3:

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).