What does the exclamation point mean in PHP?
Solution 1
!
is the logical negation or NOT
operator. It reverses the sense of the logical test.
That is:
-
if(isset)
makes something happen ifisset
is logicalTrue
. -
if(!isset)
makes something happen ifisset
is logicalFalse
.
More about operators (logical and other types) in the PHP documentation. Look up !
there to cement your understanding of what it does. While you're there, also look up the other logical operators:
-
&&
logical AND -
||
logical OR -
xor
logical EXCLUSIVE-OR
Which are also commonly used in logic statements.
Solution 2
The !
character is the logical "not" operator. It inverts the boolean meaning of the expression.
If you have an expression that evaluates to TRUE
, prefixing it with !
causes it evaluate to FALSE
and vice-versa.
$test = 'value';
var_dump(isset($test)); // TRUE
var_dump(!isset($test)); // FALSE
isset()
returns TRUE
if the given variable is defined in the current scope with a non-null value.
empty()
returns TRUE
if the given variable is not defined in the current scope, or if it is defined with a value that is considered "empty". These values are:
NULL // NULL value
0 // Integer/float zero
'' // Empty string
'0' // String '0'
FALSE // Boolean FALSE
array() // empty array
Depending PHP version, an object with no properties may also be considered empty.
The upshot of this is that isset()
and empty()
almost compliment each other (they return the opposite results) but not quite, as empty()
performs an additional check on the value of the variable, isset()
simply checks whether it is defined.
Consider the following example:
var_dump(isset($test)); // FALSE
var_dump(empty($test)); // TRUE
$test = '';
var_dump(isset($test)); // TRUE
var_dump(empty($test)); // TRUE
$test = 'value';
var_dump(isset($test)); // TRUE
var_dump(empty($test)); // FALSE
Solution 3
$var = 0;
// Evaluates to true because $var is empty
if (empty($var)) {
echo '$var is either 0, empty, or not set at all';
}
// Evaluates as true because $var is set
if (isset($var)) {
echo '$var is set even though it is empty';
}
Edit:
here is a test case for you:
$p = false;
echo isset($p) ? '$p is setted : ' : '$p is not setted : ';
echo empty($p) ? '$p is empty' : '$p is not empty';
echo "<BR>";
$p is setted : $p is empty
oscar
Updated on December 10, 2020Comments
-
oscar over 3 years
Possible Duplicate:
Reference - What does this symbol mean in PHP?
Getting confused with empty, isset, !empty, !issetIn PHP what is the difference between:
if(!isset) if(isset)
Same with
if(!empty)
andif(empty)
?What does the "!" character mean?
-
Li-aung Yip about 12 yearsString
'0'
evaluates to logical false in PHP? Ugh. :( -
DaveRandom about 12 years@Li-aungYip I know it seems illogical but it is useful. Because PHP is very loosely typed, it makes sense when you consider that one is often working with data from
$_POST
and friends, where all data is(string)
. In this instance, it is useful to be able to pass integers as booleans, and in order for this to work without explicit casting,'0'
must evaluate toFALSE
. Where it gets confusing is the fact that casting a string to int usually results in0
, yet casting as bool results inTRUE
, so(bool) $str != (bool) (int) $str
-
Li-aung Yip about 12 yearsIt's a useful idiom because it's easy - but type coercion always makes me a bit queasy because of the potential bugs. A typical pattern for me is to test the emptiness of a string by asking
if(string)
. In PHP both the empty string''
and the literal'0'
would pass this test, so I'd have to unlearn that habit. -
oscar about 12 yearsThanks, the example made a lot of sense
-
Li-aung Yip about 12 yearsAt least PHP isn't Javascript. Wat. (See explanations on SO.)
-
DaveRandom about 12 years@Li-aungYip You see I am a PHP native so I am already is the habit of
===
ing everything, and as result I never run into those interesting little ECMA oddities you pointed either - but I can see how it would be annoying for those coming from a strongly typed background.