Logical Operators

Logical Operators
Example Name Result
$a and $b And TRUE if both $a and $b are TRUE.
$a or $b Or TRUE if either $a or $b is TRUE.
$a xor $b Xor TRUE if either $a or $b is TRUE, but not both.
! $a Not TRUE if $a is not TRUE.
$a && $b And TRUE if both $a and $b are TRUE.
$a || $b Or TRUE if either $a or $b is TRUE.

The reason for the two different variations of "and" and "or" operators is that they operate at different precedences. (See Operator Precedence.)

Example #1 Logical operators illustrated

<?php

// --------------------
// foo() will never get called as those operators are short-circuit

$a = (false && foo());
$b = (true  || foo());
$c = (false and foo());
$d = (true  or  foo());

// --------------------
// "||" has a greater precedence than "or"

// The result of the expression (false || true) is assigned to $e
// Acts like: ($e = (false || true))
$e false || true;

// The constant false is assigned to $f and then true is ignored
// Acts like: (($f = false) or true)
$f false or true;

var_dump($e$f);

// --------------------
// "&&" has a greater precedence than "and"

// The result of the expression (true && false) is assigned to $g
// Acts like: ($g = (true && false))
$g true && false;

// The constant true is assigned to $h and then false is ignored
// Acts like: (($h = true) and false)
$h true and false;

var_dump($g$h);
?>

Exemplul de mai sus va afișa ceva similar cu:

bool(true)
bool(false)
bool(false)
bool(true)
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User Contributed Notes 12 notes

up
59
Lawrence
7 years ago
Note that PHP's boolean operators *always* return a boolean value... as opposed to other languages that return the value of the last evaluated expression.

For example:

$a = 0 || 'avacado';
print "A: $a\n";

will print:

A: 1

in PHP -- as opposed to printing "A: avacado" as it would in a language like Perl or JavaScript.

This means you can't use the '||' operator to set a default value:

$a = $fruit || 'apple';

instead, you have to use the '?:' operator:

$a = ($fruit ? $fruit : 'apple');
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4
anatoliy at ukhvanovy dot name
6 months ago
If you want to use the '||' operator to set a default value, like this:

<?php
$a
= $fruit || 'apple'; //if $fruit evaluates to FALSE, then $a will be set to TRUE (because (bool)'apple' == TRUE)
?>

instead, you have to use the '?:' operator:

<?php
$a
= ($fruit ? $fruit : 'apple');//if $fruit evaluates to FALSE, then $a will be set to 'apple'
?>

But $fruit will be evaluated twice, which is not desirable. For example fruit() will be called twice:
<?php
function fruit($confirm) {
    if(
$confirm)
        return
'banana';
}
$a = (fruit(1) ? fruit(1) : 'apple');//fruit() will be called twice!
?>

But since «since PHP 5.3, it is possible to leave out the middle part of the ternary operator» (http://www.php.net/manual/en/language.operators.comparison.php#language.operators.comparison.ternary), now you can code like this:

<?php
$a
= ($fruit ? : 'apple'); //this will evaluate $fruit only once, and if it evaluates to FALSE, then $a will be set to 'apple'
?>

But remember that a non-empty string '0' evaluates to FALSE!

<?php
$fruit
= '1';
$a = ($fruit ? : 'apple'); //this line will set $a to '1'
$fruit = '0';
$a = ($fruit ? : 'apple'); //this line will set $a to 'apple', not '0'!
?>
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18
pepesantillan at gmail dot com
6 years ago
worth reading for people learning about php and programming: (adding extras <?php ?> to get highlighted code)

about the following example in this page manual:
Example#1 Logical operators illustrated

...
<?php
// "||" has a greater precedence than "or"
$e = false || true; // $e will be assigned to (false || true) which is true
$f = false or true; // $f will be assigned to false
var_dump($e, $f);

// "&&" has a greater precedence than "and"
$g = true && false; // $g will be assigned to (true && false) which is false
$h = true and false; // $h will be assigned to true
var_dump($g, $h);
?>
_______________________________________________end of my quote...

If necessary, I wanted to give further explanation on this and say that when we write:
$f = false or true; // $f will be assigned to false
the explanation:

"||" has a greater precedence than "or"

its true. But a more acurate one would be

"||" has greater precedence than "or" and than "=", whereas "or" doesnt have greater precedence than "=", so

<?php
$f
= false or true;

//is like writting

($f = false ) or true;

//and

$e = false || true;

is the same as

$e = (false || true);

?>

same goes for "&&" and "AND".

If you find it hard to remember operators precedence you can always use parenthesys - "(" and ")". And even if you get to learn it remember that being a good programmer is not showing you can do code with fewer words. The point of being a good programmer is writting code that is easy to understand (comment your code when necessary!), easy to maintain and with high efficiency, among other things.
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12
phpnet at zc dot webhop dot net
2 years ago
This works similar to javascripts short-curcuit assignments and setting defaults. (e.g.  var a = getParm() || 'a default';)

<?php

($a = $_GET['var']) || ($a = 'a default');

?>

$a gets assigned $_GET['var'] if there's anything in it or it will fallback to 'a default'
Parentheses are required, otherwise you'll end up with $a being a boolean.
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9
momrom at freenet dot de
5 years ago
Evaluation of logical expressions is stopped as soon as the result is known.
If you don't want this, you can replace the and-operator by min() and the or-operator by max().

<?php
function a($x) { echo 'Expression '; return $x; }
function
b($x) { echo 'is '; return $x; }
function
c($x) { echo $x ? 'true.' : 'false.' ;}

c( a( false ) and b( true ) ); // Output: Expression false.
c( min( a( false ), b( true ) ) ); // Output: Expression is false.

c( a( true ) or b( true ) ); // Output: Expression true.
c( max( a( true ), b( true ) ) ); // Output: Expression is true.
?>

This way, values aren't automaticaly converted to boolean like it would be done when using and or or. Therefore, if you aren't sure the values are already boolean, you have to convert them 'by hand':

<?php
c
( min( (bool) a( false ), (bool) b( true ) ) );
?>
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3
brian at zickzickzick dot com
1 year ago
This has been mentioned before, but just in case you missed it:

<?php
   
// Defaults --

    //If you're trying to gat 'Jack' from:
   
$jack = false or 'Jack';

   
// Try:
   
$jack = false or $jack = 'Jack';

   
//The other option is:
   
$jack = false ? false : 'Jack';
?>
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4
peter dot kutak at NOSPAM dot gmail dot com
7 years ago
$test = true and false;     ---> $test === true
$test = (true and false);  ---> $test === false
$test = true && false;      ---> $test === false

NOTE: this is due to the first line actually being

($test = true) and false;

due to "&&" having a higher precedence than "=" while "and" has a lower one
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4
Andrew
7 years ago
> <?php
> your_function() or return "whatever";
>
?>

doesn't work because return is not an expression, it's a statement. if return was a function it'd work fine. :/
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0
tekreme73 at hotmail dot fr
7 months ago
// Operators &= and |= work as well

// "a &= b"  <=>  "a = a && b" <=> "a = a AND b"
// "a |= b"  <=>  "a = a || b" <=> "a = a OR b"

$res = true;
var_dump($res);

$res &= false;
var_dump($res);

$res |= true;
var_dump($res);

// This code will display :
bool(true)
bool(false)
bool(true)
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-2
dartello at gmail dot com
2 years ago
Unlike in C/C++ the invertor (Not) in PHP assumes a string:

<?php
$a
= 1;

$t = !$a;
var_dump($t);
?>

The above example will output:
string(0) => ""

To approach the C/C++ handling, this can be solved as follows:

<?php
$a
= 1;

(int)
$t = !$a;
var_dump($t);

(bool)
$u = !a;
var_dump($u);
?>

The above example will output:
int(0)
bool(false)
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-1
void at informance dot info
8 months ago
To assign default value in variable assignation, the simpliest solution to me is:

<?php
$v
= my_function() or $v = "default";
?>

It works because, first, $v is assigned the return value from my_function(), then this value is evaluated as a part of a logical operation:
* if the left side is false, null, 0, or an empty string, the right side must be evaluated and, again, because 'or' has low precedence, $v is assigned the string "default"
* if the left side is none of the previously mentioned values, the logical operation ends and $v keeps the return value from my_function()

This is almost the same as the solution from [phpnet at zc dot webhop dot net], except that his solution (parenthesis and double pipe) doesn't take advantage of the "or" low precedence.

NOTE: "" (the empty string) is evaluated as a FALSE logical operand, so make sure that the empty string is not an acceptable value from my_function(). If you need to consider the empty string as an acceptable return value, you must go the classical "if" way.
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-7
paranoiq at centrum dot cz
7 years ago
and, or and xor can be used as conditional constructs:

<?php
// do_that() is executed only if do_this() returns false
if($something) do_this() or do_that();
// $b is assigned to $b, do_that() is executed if $b is false
if($something) $a = $b or do_that();

// do_that() is executed only if do_this() returns true
if($something) do_this() and do_that();
// $b is assigned to $b, do_that() is executed if $b is true
if($something) $a = $b and do_that();

// both do_that() and do_this() are executed..
if($something) do_this() xor do_that();
// .. so the behaviour is same as:
if($something) {
   
do_this();
   
do_that();
}
?>

for understanding what happens if $b is NULL or do_this() returns NULL, read the avbentem's comment on NULL type. generaly speaking, NULL is threated like false in most cases.
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