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和字符串运算符的“组和运算符”，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。

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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 (+=, *=...).

trosos at atlas dot cz
03-Jul-2004 09:19


Note that the value of an assignment expression is not the value assigned, but the variable itself (after assign, of course). This allows to do something like this:



<?php



function f(&$a)

{

 ...

}



f($v=3);



?>

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

jeronimo at DELETE_THIS dot transartmedia dot com
29-Jan-2004 11:27


If you want to swap values between variables without using an intermediary, try using the list() and array() language constructs. For instance:



<?



// Initial values.

$biggest = 1;

$smallest = 10;



// Instead of using a temporary variable...

$temp = $biggest;

$biggest = $smallest;

$smallest = $temp;



// ...Just swap the values.

list($biggest, $smallest) = array($smallest, $biggest);



?>



This works with any number of variables; you're not limited to just swapping two.

Cheers,

Jeronimo

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