PHP: 赋值运算符

Last updated: Mon, 26 Nov 2007

赋值运算符

基本的赋值运算符是“=”。一开始可能会以为它是“等于”，其实不是的。它实际上意味着把右边表达式的值赋给左边的运算数。

赋值运算表达式的值也就是所赋的值。也就是说，“$a = 3”的值是 3。这样就可以做一些小技巧：


<?php

$a = ($b = 4) + 5; // $a 现在成了 9，而 $b 成了 4。

?>

在基本赋值运算符之外，还有适合于所有二元算术，数组集合和字符串运算符的“组合运算符”，这样可以在一个表达式中使用它的值并把表达式的结果赋给它，例如：


<?php

$a = 3;
$a += 5; // sets $a to 8, as if we had said: $a = $a + 5;
$b = "Hello ";
$b .= "There!"; // sets $b to "Hello There!", just like $b = $b . "There!";

?>

注意赋值运算将原变量的值拷贝到新变量中（传值赋值），所以改变其中一个并不影响另一个。这也适合于在很密集的循环中拷贝一些值例如大数组。自 PHP 4 起支持引用赋值，用 $var = &$othervar; 语法，但在 PHP 3 中不可能这样做。引用赋值意味着两个变量都指向同一个数据，没有任何数据的拷贝。有关引用的更多信息见引用的说明。

位运算符

算术运算符

Last updated: Mon, 26 Nov 2007

add a note User Contributed Notes
赋值运算符

Paul Ebermann
29-Apr-2008 09:07


There is also a array-append-Operator:



<?php

   $array []= $element;

?>



This appends the element to the end of the array, as

<?php

   array_push($array, $element);

?>

would do.

(This is documented on the array_push page, but not here in the operator section.)

Hayley Watson
06-Feb-2008 09:54


You could also take adam at gmail dot com's xor-assignment operator and use the fact that it's right-associative:



$a ^= $b ^= $a ^= $b;

Hayley Watson
08-Oct-2007 06:22


bradlis7 at bradlis7 dot com's description is a bit confusing. Here it is rephrased.



<?php

$a = 'a';

$b = 'b';



$a .= $b .= "foo";



echo $a,"\n",$b;?>

outputs



abfoo

bfoo



Because the assignment operators are right-associative and evaluate to the result of the assignment

<?php

$a .= $b .= "foo";

?>

is equivalent to

<?php

$a .= ($b .= "foo");

?>

and therefore

<?php

$b .= "foo";

$a .= $b;

?>

adam at gmail dot com
26-Aug-2006 01:38


or you could use the xor-assignment operator..

$a ^= $b;

$b ^= $a;

$a ^= $b;

bradlis7 at bradlis7 dot com
15-Aug-2005 11:13


Note whenever you do this



<?php

$a .= $b .= "bla bla";

?>



it comes out to be the same as the following:



<?php

$a .= $b."bla bla";

$b .= "bla bla";

?>



So $a actually becomes $a and the final $b string. I'm sure it's the same with numerical assignments (+=, *=...).

straz at mac dot nospam dot com
21-Feb-2004 02:18


This page really ought to have table of assignment operators,

namely,



See the Arithmetic Operators page (http://www.php.net/manual/en/language.operators.arithmetic.php)

Assignment    Same as:

$a += $b     $a = $a + $b    Addition

$a -= $b     $a = $a - $b     Subtraction

$a *= $b     $a = $a * $b     Multiplication

$a /= $b     $a = $a / $b    Division

$a %= $b     $a = $a % $b    Modulus



See the String Operators page(http://www.php.net/manual/en/language.operators.string.php)

$a .= $b     $a = $a . $b       Concatenate



See the Bitwise Operators page (http://www.php.net/manual/en/language.operators.bitwise.php)

$a &= $b     $a = $a & $b     Bitwise And

$a |= $b     $a = $a | $b      Bitwise Or

$a ^= $b     $a = $a ^ $b       Bitwise Xor

$a <<= $b     $a = $a << $b     Left shift

$a >>= $b     $a = $a >> $b      Right shift

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位运算符

算术运算符

Last updated: Mon, 26 Nov 2007