International PHP Conference 2019 - Spring Edition

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。

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User Contributed Notes 7 notes

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209
Anonymous
14 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
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20
yasuo_ohgaki at hotmail dot com
17 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 $str.
(PHP4.0.4pl1/Apache DSO/Linux, PHP4.0.5RC1/Apache DSO/W2K Server)
It's due to precedence, probably.
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0
figroc at gmail dot com
10 years ago
The variable symbol '$' 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]
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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' :
        (
$a == 4 ? 'four' : 'other') ) ) );
echo
"\n";
//prints 'two'
?>
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-26
me at robrosenbaum dot com
11 years ago
The scope resolution operator ::, which is missing from the list above, has higher precedence than [], and lower precedence than 'new'. This means that self::$array[$var] works as expected.
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-30
golotyuk at gmail dot com
12 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
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-37
rick at nomorespam dot fourfront dot ltd dot uk
13 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.
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