php 预定义变量

对于全部脚本而言，PHP 提供了大量的预定义变量。这些变量将所有的外部变量表示成内建环境变量，并且将错误信息表示成返回头。

PHP 选项 register_globals 对我有什么影响？

首先要理解这个设置选项的作用。假如我们使用以下URL： http://example.com/foo.php?animal=cat，那么在 foo.php中我们可能会使用以下代码：

<?php
// 建议使用这种访问变量的方式
echo $_GET['animal'];
// 如果想直接访问$animal变量，就要把register_globals选项设置为on
// 强烈建议不要这么做！！
echo $animal;
// 这个选项的值会影响到所有变量，包括$_SERVER
// 以下是该选项设置为off时的正确写法
echo $_SERVER['PHP_SELF'];
// 同样，要使$PHP_SELF变量自动生效，register_globals选项必须设置为on
// 强烈建议不要这么做！！
echo $PHP_SELF;
?>

上面的代码解释了register_globals的作用，就是自动生成变量。这种编程方式被很多人所不喜，所以在PHP当中register_globals默认设置为off。在PHP6，这个选项被删除了。所以目前绝大部分虚拟主机都默认把register_globals禁用。请注意，你在阅读过时的文章、教程时可能需要把该选项开启，否则可能示例代码不能通过。但在实际开发当中，强烈建议将其设置为off，否则可能会导致安全问题。

Warning: $_SERVER['PHP_SELF'] can include arbitrary user input. The documentation should be updated to reflect this.
The request "http://example.com/info.php/attack%20here" will run /info.php, but in Apache $_SERVER['PHP_SELF'] will equal "/info.php/attack here". This is a feature, but it means that PHP_SELF must be treated as user input.
The attack string could contain urlencoded HTML and JavaScript (cross-site scripting) or it could contain urlencoded linebreaks (HTTP response-splitting).
The use of $_SERVER['SCRIPT_NAME'] is recommended instead.

Re: You can take advantage of 404 error to an usable redirection using REQUEST_URI ...
Whilst this is effective, a line in the .htaccess such as:
RewriteEngine On
RewriteRule ^profiles/([A-Za-z0-9-]+) showprofile.php?profile=$1 [L,NC,QSA]
will throw the requested profile in a variable $profile to the showprofile.php page. 
You can further enhance the url (e.g http://servername/profiles/Jerry/homeaddress/index.htm) and the second variable value homeaddress becomes available in $url_array[3] when used below $url_array=explode("/",$_SERVER['REQUEST_URI']);  
Hope this helps - Works well for me
Drew

Also on using IPs to look up country & city, note that what you get might not be entirely accurate. If their ISP is based in a different city or province/state, the IPs may be owned by the head office, and used across several areas. 
You also have rarer situations where they might be SSHed into another server, on the road, at work, at a friend's... It's a nice idea, but as the example code shows, it should only be used to set defaults.

Refer to CanonicalName if you are not getting the ServerName in the $_SERVER[SERVER_NAME] variable....This was a pain to figure out for me...now it works as expected by turning canonical naming on.
http://www.apacheref.com/ref/http_core/UseCanonicalName.html

@White-Gandalf: one can control this behavior by setting:
UseCanonicalName On|Off
in their apache config (at least, on *ix platforms).
On => $_SERVER['SERVER_NAME'] is the ServerName var from either the global server or virtual host, whichever wraps the PHP app closest.
Off => Whatever was in the Host: header sent by the client.

Running PHP 4.3 under IIS 5 on Windows XP, there is no $_SERVER['REQUEST_URI'] variable. This seems to fix it:
if(!isset($_SERVER['REQUEST_URI'])) {
  $_SERVER['REQUEST_URI'] = substr($_SERVER['argv'][0], strpos($_SERVER['argv'][0], ';') + 1);
}

I think it is very important to note that PHP will automatically replace dots ('.') AND spaces (' ') with underscores ('_') in any incoming POST or GET (or REQUEST) variables.
This page notes the dot replacement, but not the space replacement:
http://us2.php.net/manual/en/language.variables.external.php
The reason is that '.' and ' ' are not valid characters to use in a variable name. This is confusing to many people, because most people use the format $_POST['name'] to access these values. In this case, the name is not used as a variable name but as an array index, in which those characters are valid.
However, if the register_globals directive is set, these names must be used as variable names. As of now, PHP converts the names for these variables before inserting them into the external variable arrays, unfortunately - rather than leaving them as they are for the arrays and changing the names only for the variables set by register_globals.
If you want to use:
<input name="title for page3.php" type="text">
The value you will get in your POST array, for isntance would be:
$_POST['title_for_page3_php']

Nothing about the message-body ...
You can get cookies, session variables, headers, the request-uri , the request method, etc but not the message body. You may want it sometimes when your page is to be requested with the POST method.
Maybe they should have mentioned $HTTP_RAW_POST_DATA or php://stdin

The Environment variable $ENV is useful for coding portable platform specific application constants. 
// Define a Windows or else Linux root directory path
$_ENV['OS'] == 'Windows_NT' ? $path = 'L:\\www\\' : $path = ' /var/www/';
define('PATH', $path);
echo PATH;

$_GET may not handle query string parameter values which include escaped Unicode values resulting from applying the JavaScript "escape" function to a Unicode string.
To handle this the query parameter value can be obtained using a function such as:
function getQueryParameter ($strParam) {
 $aParamList = explode('&', $_SERVER['QUERY_STRING']);
 $i = 0;
 while ($i < count($aParamList)) {
  $aParam = split('=', $aParamList[$i]);
  if ($strParam == $aParam[0]) {
   return $aParam[1];
  } 
 }
 return "";
}
or by directly building an array or query string values and then processing the parameter string using a function such as the "unescape" function which can be found at http://www.kanolife.com/escape/2006/03/unicode-url-escapes-in-php.html (or http://www.kanolife.com/escape/ for related info).

So you have an application in your web space, with a URL such as this:
http://<host>/<installation_path>/
and pages such as
http://<host>/<installation_path>/subfolder1/subfolder2/page.php
You have a file called config.php in <installation_path> which is include()d by all pages (in subfolders or not).
How to work out <installation_path> without hard-coding it into a config file? 
<?php
// this is config.php, and it is in <installation_path>
// it is included by <installation_path>/page.php
// it is included by <installation_path>/subfolder/page2.php
// etc
$_REAL_SCRIPT_DIR = realpath(dirname($_SERVER['SCRIPT_FILENAME'])); // filesystem path of this page's directory (page.php)
$_REAL_BASE_DIR = realpath(dirname(__FILE__)); // filesystem path of this file's directory (config.php)
$_MY_PATH_PART = substr( $_REAL_SCRIPT_DIR, strlen($_REAL_BASE_DIR)); // just the subfolder part between <installation_path> and the page
$INSTALLATION_PATH = $_MY_PATH_PART
  ? substr( dirname($_SERVER['SCRIPT_NAME']), 0, -strlen($_MY_PATH_PART) )
  : dirname($_SERVER['SCRIPT_NAME'])
; // we subtract the subfolder part from the end of <installation_path>, leaving us with just <installation_path> :)
?>

I use HTTP_X_FORWARDED_FOR because my webserver is behind a reverse proxy.
This can be made secure:
Configure the reverse proxy to block this field, and override it correctly.
Configure the apache server to only accept incoming connections from the reverse proxy.

If you use Apache's redirection features for custom error pages or whatever, the following Apache's REDIRECT variables are also available in $_SERVER:
$_SERVER['REDIRECT_UNIQUE_ID]' 
$_SERVER['REDIRECT_SCRIPT_URL]' 
$_SERVER['REDIRECT_SCRIPT_URI]' 
$_SERVER['REDIRECT_SITE_ROOT]' 
$_SERVER['REDIRECT_SITE_HTMLROOT]' 
$_SERVER['REDIRECT_SITE_CGIROOT]' 
$_SERVER['REDIRECT_STATUS]' 
$_SERVER['REDIRECT_QUERY_STRING]' 
$_SERVER['REDIRECT_URL]' 
I'm not sure if this is a complete list though

if you try to run php through command line, for example: php.exe c:\AppServ\www\cron_cache.php. You better avoid to use $_SERVER['DOCUMENT_ROOT'], because it will return nothing. Instead, you can use dirname(__FILE__). The reason to use command line running php is set it as Windows Scheduled Tasks. I did not test under Linux environment, but might be same.

Be carful when using $_SERVER['DOCUMENT_ROOT']; in your applications where you want to distribute them to other people with different server types. It isnt always supported by the webserver (IIS).

SECURITY RISK !
Never ever trust the values that comes from $_SERVER.
HTTP_X_FORWARDED, HTTP_X_FORWARDED_FOR, HTTP_FORWARDED_FOR, HTTP_FORWARDED, etc.. can be spoofed !
To get the ip of user, use only $_SERVER['REMOTE_ADDR'], otherwise the 'ip' of user can be easily changed by sending a HTTP_X_* header, so user can escape a ban or spoof a trusted ip.
Of course this is well know, but I don't see it mentioned in these notes..
If you use the ip only for tracking (not for any security features like banning or allow access to something by ip), you can also use HTTP_X_FORWARDED to get user's ip what are behind proxy.

If you have problems with $_SERVER['HTTPS'], especially if it returns no values at all you should check the results of phpinfo(). It might not be listed at all. 
Here is a solution to check and change, if necessary, to ssl/https that will work in all cases:
<?php
if ($_SERVER['SERVER_PORT']!=443) {
$sslport=443; //whatever your ssl port is
$url = "https://". $_SERVER['SERVER_NAME'] . ":" . $sslport . $_SERVER['REQUEST_URI'];
header("Location: $url");
}
?>
Of course, this should be done before any html tag or php echo/print.

Note that $php_errormsg may contain a newline character. This can be problematic if you are trying to output it with a JavaScript "alert()" for example.

In addition to mfyahya at gmail dot com (2007-06-07 03:33):
If You are working with the Apache module mod_rewrite and want to set some environment vars, the Apache manual says this vars could be accessed in CGI using $ENV{VAR}. In PHP You might want to write $_ENV['VAR'] to get the value of VAR, but You have to access if via $_SERVER, and in some different ways:
1. Example: .htaccess and example.php
RewriteEngine on
RewriteRule ^?var1=([^;]*);var2=([^;]*)$ \
 - [E=VAR1:$1,E=VAR2:$2]
<?php echo($_SERVER['VAR1']."\r\n"
     .$_SERVER['VAR2']); ?>
2. Example: .htaccess and index.php
RewriteEngine on
RewriteRule ^index\.php$ - [L]
RewriteRule ?var1=([^;]*);var2=([^;]*)$ \
 index.php [E=VAR1:$1,E=VAR2:$2]
<?php echo($_SERVER['REDIRECT_VAR1']."\r\n"
     .$_SERVER['REDIRECT_VAR2']); ?>
Note: If any RewriteRule matches, an internal redirect than restarts (after the last defined rule, or immediately after the matched rule having a L-flag) checking the entire rule set again. For an internal redirect every defined VAR gets an 'REDIRECT_' prefix, i.e. VAR1 will be REDIRECT_VAR1, VAR2 will be REDIRECT_VAR2.
Of course, You can (additionally) redefine the original VAR:
RewriteEngine on
RewriteRule ^index\.php$ \
 - [E=VAR1:%{REDIRECT_VAR1},E=VAR2:%{REDIRECT_VAR2},L]
RewriteRule ?var1=([^;]*);var2=([^;]*)$ \
 index.php [E=VAR1:$1,E=VAR2:$2]
With this, You will have $_SERVER['REDIRECT_VAR*'] -and- $_SERVER['VAR*'].
***
The given examples are only for explanation, in any case they are not intended to fit Your needs. The "\<CRLF><SP>" in the .htaccess examples are only for display purpose, they should not occur in a real .htaccess file. The argument separator ';' in links can also be '&', but this may cause some trouble with HTML/XHTML. See the following pages for more information about this issue:
- http://www.w3.org/TR/html4/appendix/notes.html#h-B.2.2
- http://www.w3.org/QA/2005/04/php-session

I was unable to convince my hosting company to change their installation of PHP and therefore had to find my own way to computer $_SERVER["DOCUMENT_ROOT"]. I eventually settled on the following, which is a combination of earlier notes (with some typos corrected):
<?php
if ( ! isset($_SERVER['DOCUMENT_ROOT'] ) )
 $_SERVER['DOCUMENT_ROOT'] = str_replace( '\\', '/', substr(
  $_SERVER['SCRIPT_FILENAME'], 0, 0-strlen($_SERVER['PHP_SELF']) ) );
?>

To convert query string parameter values ($_GET, $_REQUEST), which include escaped Unicode values resulting from applying the JavaScript "escape" function to a Unicode string (%uNNNN%uNNNN%uNNNN) fast and simple is to use PECL JSON extension:
function JavaScript_Unicode_URL_2_Str($js_uni_str) {
    $res = preg_replace('/%u([[:alnum:]]{4})/', '\\u\1', $js_uni_str);
    $res = str_replace('"', '\"', $res); // if in str "
    $res = json_decode('["'.$res.'"]'); // JavaScrip array with string element
    $res = $res[0];
    $res = iconv('UTF-8', ini_get('default_charset'), $res);
    return $res;
  }

The variable $php_errormsg is not populated if you have XDebug running.

I was a little frustrated by the fact that some of the _SERVER variables didn't seem to exist, so I did a bit of Googling and found the answer: many of these variables are supplied by the web server and not all web servers supply the same set of variables.
I found a comparison between Apache v1.3.29 and IIS v5.1 on this page: http://koivi.com/apache-iis-php-server-array.php Useful for those of us doing cross-platform development.
While running experiments with different browsers I noticed some of the HTTP_* variables come and go depending on the browser used, or in the case of Opera by diddling the "user mode" (the widget that lets you look at a page as text only, etc.). For example: in IE and Opera HTTP_KEEP_ALIVE was missing, but was present in Firefox and Mozilla, and when I fiddled with Opera's "user mode" I got somethings called HTTP_TE and HTTP_CACHE_CONTROL.
So, what you get is dependent on the web server AND the browser.
I did see one IIS supplied variable not on that list: REQUEST_TIME, which seems to be in Unix timestamp format.
While researching this I discovered there are plenty of people who have their phpinfo() page visible and indexed on a few search engines. For those who want to dig a bit deeper than that nice web page comparing Apache to IIS, looking at other peoples' phpinfo() pages could be useful. You get the version of PHP plus OS and web server they use, along with all the _SERVER variables. I found the highest percent of signal-to-noise by searching for "phpinfo()" (with the quotes) on Dogpile: http://www.dogpile.com/

$_SERVER['QUERY_STRING'] 
Does not contain XHTML 1.1 compliant ampersands i.e. &amp;
So you will need to do something like this if you are to use $_SERVER['QUERY_STRING'] in URL's.
// XHTML 1.1 compliant ampersands 
$_SERVER['QUERY_STRING'] = 
str_replace(array('&amp;', '&'), array('&', '&amp;'), 
$_SERVER['QUERY_STRING']);

Note that it's a very, very bad idea to append to global variables in a loop, unless you really, really mean to do so in a global context. I just a while ago hung my server with a snippet of code like this:
<?php
$host = $_SERVER['HTTP_HOST'];
$uri = rtrim($_SERVER['PHP_SELF'], "/\\");
  
$GLOBALS['SITE_ROOT'] = "http://$host$uri";
while ($i < somenumber)
readfile($GLOBALS['SITE_ROOT'] = $GLOBALS['SITE_ROOT'] . '/this/file.php');
$i++
}
?>
While it is an entertaining and unusual method of creating very long URLs and breaking servers, it's a pretty awesomely bad idea 
(Especially considering that the script in question ran concurrently with others of it's type, so the value in $GLOBALS['SITE_ROOT'] was unknown.)