类型戏法
$name = 'Some_text';
$$name = 123;
echo $Some_text; // outputs: 123
That means practically that variable $$name
is named after a string.
变量
Table of Contents
基础
PHP 中的变量用一个美元符号后面跟变量名来表示。变量名是区分大小写的。
变量名与 PHP 中其它的标签一样遵循相同的规则。一个有效的变量名由字母或者下划线开头，后面跟上任意数量的字母，数字，或者下划线。按照正常的正则表达式，它将被表述为：'[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*'。
Note: 在此所说的字母是 a-z，A-Z，以及 ASCII 字符从 127 到 255（0x7f-0xff）。
有关变量的函数信息见变量函数。
<?php
$var = 'Bob';
$Var = 'Joe';
echo "$var, $Var"; // 输出 "Bob, Joe"
$4site = 'not yet'; // 非法变更名；以数字开头
$_4site = 'not yet'; // 合法变量名；以下划线开头
$i站点is = 'mansikka'; // 合法变量名；可以用中文
?>
PHP 3 中，变量总是传值赋值。那也就是说，当将一个表达式的值赋予一个变量时，整个原始表达式的值被赋值到目标变量。这意味着，例如，当一个变量的值赋予另外一个变量时，改变其中一个变量的值，将不会影响到另外一个变量。有关这种类型的赋值操作，请参阅表达式一章。
PHP 4 提供了另外一种方式给变量赋值：引用赋值。这意味着新的变量简单的引用（换言之，“成为其别名” 或者 “指向”）了原始变量。改动新的变量将影响到原始变量，反之亦然。
使用引用赋值，简单地将一个 & 符号加到将要赋值的变量前（源变量）。例如，下列代码片断将输出“My name is Bob”两次：
<?php
$foo = 'Bob'; // Assign the value 'Bob' to $foo
$bar = &$foo; // Reference $foo via $bar.
$bar = "My name is $bar"; // Alter $bar...
echo $bar;
echo $foo; // $foo is altered too.
?>
有一点重要事项必须指出，那就是只有有名字的变量才可以引用赋值。
<?php
$foo = 25;
$bar = &$foo; // This is a valid assignment.
$bar = &(24 * 7); // Invalid; references an unnamed expression.
function test()
{
return 25;
}
$bar = &test(); // Invalid.
?>
虽然在 PHP 中并不需要初始化变量，但这是个好习惯。未初始化的变量具有其类型的默认值 - FALSE，零，空字符串或者空数组。
Example#1 未初始化变量的默认值
<?php
echo ($unset_bool ? "true" : "false"); // false
$unset_int += 25; // 0 + 25 => 25
echo $unset_string . "abc"; // "" . "abc" => "abc"
$unset_array[3] = "def"; // array() + array(3 => "def") => array(3 => "def")
?>
依赖未初始化变量的默认值在某些情况下会有问题，例如把一个文件包含到另一个之中时碰上相同的变量名。另外把 register_globals 打开是一个主要的安全隐患。E_NOTICE 级别的错误会在碰上未初始化的变量时发出，但是在向一个未初始化的数组附加单元时不会。isset() 语言结构可以用来检测一个变量是否已被初始化。
add a note
User Contributed Notes变量
panosperseas at yahoo dot com (Panagiotis Gr)
26-Aug-2009 07:31
26-Aug-2009 07:31
the_good_technician [a.t.] y-a-h-o-o.
07-Aug-2009 12:32
07-Aug-2009 12:32
Sometimes it's necessary to retrieve the value of $this from within an object.
This solution dedicates a public var $ID to store a unique identifier for the object:
<?php
class Someclass {
public $ID;
public function what_is_this() {
return '$this is: ' . $this->get_varname();
}
private function get_varname() {
$this->ID = uniqid();
foreach($GLOBALS as $key => $val) {
if (is_a($val,'Someclass')) {
if ($val->ID === $this->ID) {
return $key;
break;
}
}
}
}
}
// example:
$my_var_instance = new Someclass;
echo $my_var_instance->what_is_this(); // returns '$this is: my_var_instance'
?>
mccarre at uwindsor dot canada
09-Jan-2009 04:19
09-Jan-2009 04:19
I needed a form of refelction for my work that still uses 4.2.0 that gets a variables name in context with the application.
For example
[code]class someclass{
var $field1
function getFieldsName(){
print getVarName($this->field1);
}
}
myObject = new someclass();
myObject->getFieldsName();
//This will return "myObject->field1".
[/code]
If you want to change the format you can do so with the code, my code isn't fully complete for features, but it does the basics of what is necessary for getting a variables name and it's owners.
[code]
function getVarName(&$var)
{
$old = $var; //store the value of the var so we don't lose it.
$var = "__random__" . rand() . "temp"; //assign a random string to distinguish this var from others.
//search for the random string to find the variable in $GLOBALS
//Note: this is an iterative deepening depth first search.
$key = IDDF_search(array_reverse($GLOBALS), $var); //we reverse the global array first to make it faster,
//because variables appear at the bottom of hte globals array list.
$var = $old;
return $key;
}
function IDDF_search($array, $var, $depth=100)
{
for($i=0; $i<$depth; $i++)
if($temp = RestrictedDF_search($array, $var, $i))
return $temp;
}
function RestrictedDF_search($array, $var, $depth=100, $inArray=false)
{
if($depth < 0) //we've gone to our maximum depth, so return.
return false;
$depth--;
$possibleCloseBracket = ($inArray) ? "]" : "";
foreach($array as $key => $value) {
if(is_string($value) && $value === $var)
return $key . $possibleCloseBracket;
//else
//print $key . ": the value != " . $var . "<br />";
if(is_array($value))
if($temp = RestrictedDF_search($value, $var, $depth, true))
return $key . $possibleCloseBracket . "[" . $temp;
//PHP 4.2.0 =)
if(is_object($value))
if($temp = RestrictedDF_search(get_object_vars($value), $var, $depth))
return $key . $possibleCloseBracket . "->" . $temp;
}
return false;
}
[/code]
This was built of a function noted here at an earlier date. I think he actually named his function getVarName. Mine takes it a step further combining the recursive search defined in the array_search notes as well, so a user can get the name of a variable no matter where it is in GLOBALS.
If only I had php 5 to work with at work. =P
Anonymous
20-Jul-2008 09:25
20-Jul-2008 09:25
[EDIT by danbrown AT php DOT net: The function provided by this author will give you all defined variables at runtime. It was originally written by (john DOT t DOT gold AT gmail DOT com), but contained some errors that were corrected in subsequent posts by (ned AT wgtech DOT com) and (taliesin AT gmail DOT com).]
<?php
echo '<table border=1><tr> <th>variable</th> <th>value</th> </tr>';
foreach( get_defined_vars() as $key => $value)
{
if (is_array ($value) )
{
echo '<tr><td>$'.$key .'</td><td>';
if ( sizeof($value)>0 )
{
echo '"<table border=1><tr> <th>key</th> <th>value</th> </tr>';
foreach ($value as $skey => $svalue)
{
echo '<tr><td>[' . $skey .']</td><td>"'. $svalue .'"</td></tr>';
}
echo '</table>"';
}
else
{
echo 'EMPTY';
}
echo '</td></tr>';
}
else
{
echo '<tr><td>$' . $key .'</td><td>"'. $value .'"</td></tr>';
}
}
echo '</table>';
?>
alexandre at nospam dot gaigalas dot net
07-Jul-2007 12:13
07-Jul-2007 12:13
Here's a simple solution for retrieving the variable name, based on the lucas (http://www.php.net/manual/en/language.variables.php#49997) solution, but shorter, just two lines =)
<?php
function var_name(&$var, $scope=0)
{
$old = $var;
if (($key = array_search($var = 'unique'.rand().'value', !$scope ? $GLOBALS : $scope)) && $var = $old) return $key;
}
?>
jsb17 at cornell dot edu
21-Feb-2007 12:48
21-Feb-2007 12:48
As an addendum to David's 10-Nov-2005 posting, remember that curly braces literally mean "evaluate what's inside the curly braces" so, you can squeeze the variable variable creation into one line, like this:
<?php
${"title_default_" . $title} = "selected";
?>
and then, for example:
<?php
$title_select = <<<END
<select name="title">
<option>Select</option>
<option $title_default_Mr value="Mr">Mr</option>
<option $title_default_Ms value="Ms">Ms</option>
<option $title_default_Mrs value="Mrs">Mrs</option>
<option $title_default_Dr value="Dr">Dr</option>
</select>
END;
?>
code at slater dot fr
25-Jan-2007 06:10
25-Jan-2007 06:10
Here's a pair of functions to encode/decode any string to be a valid php and javascript variable name.
<?php
function label_encode($txt) {
// add Z to the begining to avoid that the resulting
// label is a javascript keyword or it starts with a
// number
$txt = 'Z'.$txt;
// encode as urlencoded data
$txt = rawurlencode($txt);
// replace illegal characters
$illegal = array('%', '-', '.');
$ok = array('é', 'è', 'à');
$txt = str_replace($illegal,$ok, $txt);
return $txt;
}
function label_decode($txt) {
// replace illegal characters
$illegal = array('%', '-', '.');
$ok = array('é', 'è', 'à');
$txt = str_replace($ok, $illegal, $txt);
// unencode
$txt = rawurldecode($txt);
// remove the leading Z and return
return substr($txt,1);
}
?>
molnaromatic at gmail dot com
20-May-2006 08:44
20-May-2006 08:44
Simple sample and variables and html "templates":
The PHP code:
variables.php:
<?php
$SYSN["title"] = "This is Magic!";
$SYSN["HEADLINE"] = "Ez magyarul van"; // This is hungarian
$SYSN["FEAR"] = "Bell in my heart";
?>
index.php:
<?php
include("variables.php");
include("template.html");
?>
The template:
template.html
<html>
<head><title><?=$SYSN["title"]?></title></head>
<body>
<H1><?=$SYSN["HEADLINE"]?></H1>
<p><?=$SYSN["FEAR"]?></p>
</body>
</html>
This is simple, quick and very flexibile
Mike at ImmortalSoFar dot com
26-Nov-2005 06:03
26-Nov-2005 06:03
References and "return" can be flakey:
<?php
// This only returns a copy, despite the dereferencing in the function definition
function &GetLogin ()
{
return $_SESSION['Login'];
}
// This gives a syntax error
function &GetLogin ()
{
return &$_SESSION['Login'];
}
// This works
function &GetLogin ()
{
$ret = &$_SESSION['Login'];
return $ret;
}
?>
david at removethisbit dot futuresbright dot com
10-Nov-2005 05:25
10-Nov-2005 05:25
When using variable variables this is invalid:
<?php
$my_variable_{$type}_name = true;
?>
to get around this do something like:
<?php
$n="my_variable_{$type}_name";
${$n} = true;
?>
(or $$n - I tend to use curly brackets out of habit as it helps t reduce bugs ...)
Chris Hester
31-Aug-2005 08:09
31-Aug-2005 08:09
Variables can also be assigned together.
<?php
$a = $b = $c = 1;
echo $a.$b.$c;
?>
This outputs 111.
Mike Fotes
10-Jul-2005 02:46
10-Jul-2005 02:46
In conditional assignment of variables, be careful because the strings may take over the value of the variable if you do something like this:
<?php
$condition = true;
// Outputs " <-- That should say test"
echo "test" . ($condition) ? " <-- That should say test" : "";
?>
You will need to enclose the conditional statement and assignments in parenthesis to have it work correctly:
<?php
$condition = true;
// Outputs "test <-- That should say test"
echo "test" . (($condition) ? " <-- That should say test " : "");
?>
josh at PraxisStudios dot com
18-May-2005 04:06
18-May-2005 04:06
As with echo, you can define a variable like this:
<?php
$text = <<<END
<table>
<tr>
<td>
$outputdata
</td>
</tr>
</table>
END;
?>
The closing END; must be on a line by itself (no whitespace).
[EDIT by danbrown AT php DOT net: This note illustrates HEREDOC syntax. For more information on this and similar features, please read the "Strings" section of the manual here: http://www.php.net/manual/en/language.types.string.php ]
mike at go dot online dot pt
08-Apr-2005 12:18
08-Apr-2005 12:18
In addition to what jospape at hotmail dot com and ringo78 at xs4all dot nl wrote, here's the sintax for arrays:
<?php
//considering 2 arrays
$foo1 = array ("a", "b", "c");
$foo2 = array ("d", "e", "f");
//and 2 variables that hold integers
$num = 1;
$cell = 2;
echo ${foo.$num}[$cell]; // outputs "c"
$num = 2;
$cell = 0;
echo ${foo.$num}[$cell]; // outputs "d"
?>
lucas dot karisny at linuxmail dot org
15-Feb-2005 08:42
15-Feb-2005 08:42
Here's a function to get the name of a given variable. Explanation and examples below.
<?php
function vname(&$var, $scope=false, $prefix='unique', $suffix='value')
{
if($scope) $vals = $scope;
else $vals = $GLOBALS;
$old = $var;
$var = $new = $prefix.rand().$suffix;
$vname = FALSE;
foreach($vals as $key => $val) {
if($val === $new) $vname = $key;
}
$var = $old;
return $vname;
}
?>
Explanation:
The problem with figuring out what value is what key in that variables scope is that several variables might have the same value. To remedy this, the variable is passed by reference and its value is then modified to a random value to make sure there will be a unique match. Then we loop through the scope the variable is contained in and when there is a match of our modified value, we can grab the correct key.
Examples:
1. Use of a variable contained in the global scope (default):
<?php
$my_global_variable = "My global string.";
echo vname($my_global_variable); // Outputs: my_global_variable
?>
2. Use of a local variable:
<?php
function my_local_func()
{
$my_local_variable = "My local string.";
return vname($my_local_variable, get_defined_vars());
}
echo my_local_func(); // Outputs: my_local_variable
?>
3. Use of an object property:
<?php
class myclass
{
public function __constructor()
{
$this->my_object_property = "My object property string.";
}
}
$obj = new myclass;
echo vname($obj->my_object_property, $obj); // Outputs: my_object_property
?>
jospape at hotmail dot com
05-Feb-2005 03:45
05-Feb-2005 03:45
<?php
$id = 2;
$cube_2 = "Test";
echo ${cube_.$id};
// will output: Test
?>
ringo78 at xs4all dot nl
15-Jan-2005 04:27
15-Jan-2005 04:27
<?php
// I am beginning to like curly braces.
// I hope this helps for you work with them
$filename0="k";
$filename1="kl";
$filename2="klm";
$i=0;
for ($varname = sprintf("filename%d",$i); isset ( ${$varname} ) ; $varname = sprintf("filename%d", $i) ) {
echo "${$varname} <br>";
$varname = sprintf("filename%d",$i);
$i++;
}
?>
Carel Solomon
07-Jan-2005 07:02
07-Jan-2005 07:02
You can also construct a variable name by concatenating two different variables, such as:
<?php
$arg = "foo";
$val = "bar";
//${$arg$val} = "in valid"; // Invalid
${$arg . $val} = "working";
echo $foobar; // "working";
//echo $arg$val; // Invalid
//echo ${$arg$val}; // Invalid
echo ${$arg . $val}; // "working"
?>
Carel
raja shahed at christine nothdurfter dot com
26-May-2004 01:58
26-May-2004 01:58
<?php
error_reporting(E_ALL);
$name = "Christine_Nothdurfter";
// not Christine Nothdurfter
// you are not allowed to leave a space inside a variable name ;)
$$name = "'s students of Tyrolean language ";
print " $name{$$name}<br>";
print "$name$Christine_Nothdurfter";
// same
?>
webmaster at daersys dot net
21-Jan-2004 12:15
21-Jan-2004 12:15
You don't necessarily have to escape the dollar-sign before a variable if you want to output its name.
You can use single quotes instead of double quotes, too.
For instance:
<?php
$var = "test";
echo "$var"; // Will output the string "test"
echo "\$var"; // Will output the string "$var"
echo '$var'; // Will do the exact same thing as the previous line
?>
Why?
Well, the reason for this is that the PHP Parser will not attempt to parse strings encapsulated in single quotes (as opposed to strings within double quotes) and therefore outputs exactly what it's being fed with :)
To output the value of a variable within a single-quote-encapsulated string you'll have to use something along the lines of the following code:
<?php
$var = 'test';
/*
Using single quotes here seeing as I don't need the parser to actually parse the content of this variable but merely treat it as an ordinary string
*/
echo '$var = "' . $var . '"';
/*
Will output:
$var = "test"
*/
?>
HTH
- Daerion
unleaded at nospam dot unleadedonline dot net
15-Jan-2003 10:37
15-Jan-2003 10:37
References are great if you want to point to a variable which you don't quite know the value yet ;)
eg:
<?php
$error_msg = &$messages['login_error']; // Create a reference
$messages['login_error'] = 'test'; // Then later on set the referenced value
echo $error_msg; // echo the 'referenced value'
?>
The output will be:
test