运算符

Table of Contents

运算符是可以通过给出的一或多个值(用编程行话来说,表达式)来产生另一个值(因而整个结构成为一个表达式)的东西。所以可以认为函数或任何会返回一个值(例如 print)的结构是运算符,而那些没有返回值的(例如 echo)是别的东西。

有三种类型的运算符。第一种是一元运算符,只运算一个值,例如 !(取反运算符)或 ++(加一运算符)。第二种是有限二元运算符,PHP 支持的大多数运算符都是这种。列表见下节运算符优先级

第三种是三元运算符:?:。它应该被用来根据一个表达式在另两个表达式中选择一个,而不是用来在两个语句或者程序路线中选择。把整个三元表达式放在扩号里是个很好的主意。

add a note add a note

User Contributed Notes 13 notes

up
203
Anonymous
13 years ago
of course this should be clear, but i think it has to be mentioned espacially:

AND is not the same like &&

for example:

<?php $a && $b || $c; ?>
is not the same like
<?php $a AND $b || $c; ?>

the first thing is
(a and b) or c

the second
a and (b or c)

'cause || has got a higher priority than and, but less than &&

of course, using always [ && and || ] or [ AND and OR ] would be okay, but than you should at least respect the following:

<?php $a = $b && $c; ?>
<?php $a
= $b AND $c; ?>

the first code will set $a to the result of the comparison $b with $c, both have to be true, while the second code line will set $a like $b and THAN - after that - compare the success of this with the value of $c

maybe usefull for some tricky coding and helpfull to prevent bugs :D

greetz, Warhog
up
19
yasuo_ohgaki at hotmail dot com
16 years ago
Other Language books' operator precedence section usually include "(" and ")" - with exception of a Perl book that I have. (In PHP "{" and "}" should also be considered also). However, PHP Manual is not listed "(" and ")" in precedence list. It looks like "(" and ")" has higher precedence as it should be.

Note: If you write following code, you would need "()" to get expected value.

<?php
$bar
= true;
$str = "TEST". ($bar ? 'true' : 'false') ."TEST";
?>

Without "(" and ")" you will get only "true" in $str.
(PHP4.0.4pl1/Apache DSO/Linux, PHP4.0.5RC1/Apache DSO/W2K Server)
It's due to precedence, probably.
up
7
figroc at gmail dot com
9 years ago
The variable symbol '$' should be considered as the highest-precedence operator, so that the variable variables such as $$a[0] won't confuse the parser.  [http://www.php.net/manual/en/language.variables.variable.php]
up
0
phpnet dot 20 dot dpnsubs at xoxy dot net
10 years ago
Note that in php the ternary operator ?: has a left associativity unlike in C and C++ where it has right associativity.

You cannot write code like this (as you may have accustomed to in C/C++):

<?php
$a
= 2;
echo (
   
$a == 1 ? 'one' :
   
$a == 2 ? 'two' :
   
$a == 3 ? 'three' :
   
$a == 4 ? 'four' : 'other');
echo
"\n";
// prints 'four'
?>

You need to add brackets to get the results you want:

<?php
$a
= 2;

echo (
$a == 1 ? 'one' :
        (
$a == 2 ? 'two' :
        (
$a == 3 ? 'three' :
        (
$a == 4 ? 'four' : 'other') ) ) );
echo
"\n";
//prints 'two'
?>
up
-15
janturon at email dot cz
10 years ago
This is very common problem: set one variable to another, if it is not empty. If it is, set it to something else.
For example: set $bar to $foo, if $foo is empty, set $bar to "undefined";

if(!empty($foo)) $bar= $foo; else $bar= "undefined";

OR operator can shorten it:

$bar= @$foo or $bar= "undefined";
up
-16
golotyuk at gmail dot com
11 years ago
Simple POST and PRE incremnt sample:

<?php

$b
= 5;
$a = ( ( ++$b ) > 5 ); // Pre-increment test
echo (int)$a;

$b = 5;
$a = ( ( $b++ ) > 5 ); // Post-increment test
echo (int)$a;

?>

This will output 10, because of the difference in post- and pre-increment operations
up
-15
madcoder at gmail dot com
10 years ago
In response to mathiasrav at gmail dot com:

The reason for that behavior is the parentheses.  From the description:

"Parentheses may be used to force precedence, if necessary. For instance: (1 + 5) * 3 evaluates to 18."

So the order of operations says that even though the equality operator has higher precedence, the parentheses in your statement force the assignment operator to a higher precedence than the equality operator.

That said, it still doesn't work the way you expect it to.  Neither way works, for these reasons:
<?php
if ( $a != ($a = $b) )
?>

Order of operations says to do the parentheses first.  So you end up with:
<?php
$a
= $b;
if (
$a != $a )
?>

Which is obviously going to be false.  Without the parentheses:
<?php
if ( $a != $a = $b )
?>

Order of operations says to do the inequality first, then the assignment, so you have:
<?php
if ( $a != $a );
$a = $b;
?>

Which again is not what you expected, and again will always be false.  But because you are only working with values of 0 and 1, you can make use of the XOR operator:

<?php
if ( $a ^= $b )
?>

This will only be true if 1) $a is 0 and $b is 1, or 2) $a is 1 and $b is 0.  That is precisely what you wanted, and it even does the assignment the way you expected it to.

<?php
foreach ($ourstring as $c) {
  if (
$bold ^= $c['bold']) $resstring .= bold;
  if (
$underline ^= $c['underline']) $resstring .= underline;
 
$resstring .= $c[0];
}
?>

That code now works and produces the output you expected.
up
-21
rick at nomorespam dot fourfront dot ltd dot uk
12 years ago
A quick note to any C developers out there, assignment expressions are not interpreted as you may expect - take the following code ;-

<?php
$a
=array(1,2,3);
$b=array(4,5,6);
$c=1;

$a[$c++]=$b[$c++];

print_r( $a ) ;
?>

This will output;-
Array ( [0] => 1 [1] => 6 [2] => 3 )
as if the code said;-
$a[1]=$b[2];

Under a C compiler the result is;-
Array ( [0] => 1 [1] => 5 [2] => 3 )
as if the code said;-
$a[1]=$b[1];

It would appear that in php the increment in the left side of the assignment is processed prior to processing the right side of the assignment, whereas in C, neither increment occurs until after the assignment.
up
-24
me at robrosenbaum dot com
10 years ago
The scope resolution operator ::, which is missing from the list above, has higher precedence than [], and lower precedence than 'new'. This means that self::$array[$var] works as expected.
up
-31
pgarvin76+php dot net at NOSPAMgmail dot com
8 years ago
Method chaining is read left to right (left associative):
<?php

class Test_Method_Chain
{
    public function
One()
    {
        echo
"One" . PHP_EOL;
        return
$this;
    }

    public function
Two()
    {
        echo
"Two" . PHP_EOL;
        return
$this;
    }

    public function
Three()
    {
        echo
"Three" . PHP_EOL;
        return
$this;
    }
}

$test = new Test_Method_Chain();

$test->One()->Two()->Three();

/* Ouputs:
One
Two
Three
*/
?>
up
-29
Gautam
10 years ago
<?php

$result1
= 7 + 8 * 9/3 -4;
$result2 = 7 + 8 * (9/3 -4);
$result3 =(7 + 8)* 9/3 -4;

echo
"Result1 for 7 + 8 * 9/3 -4 = $result1  Result2 for 7 + 8 * (9/3 -4) = $result2 and Result3 (7 + 8)* 9/3 -4 = $result3 "
/*
which gives results as under
Result1 for 7 + 8 * 9/3 -4 = 27 Result2 for 7 + 8 * (9/3 -4) = -1 and Result3 (7 + 8)* 9/3 -4 = 41
Execution Order is 1) expression in brackets 2) division 3) multiplication 4) addition and 5) subtraction
*/
?>
up
-24
arth dot inbox+php dot net at gmail dot com
2 years ago
&= operator.

It is binary and followed expressions give different results:

$a = $b = 1;
echo $a &= 2;  // 0
echo $a = $a && 2; // 1
up
-105
ddascalescu at gmail dot com
9 years ago
The -> operator, not listed above, is called "object operator" (T_OBJECT_OPERATOR).
To Top