You want to get the value of an environment variable.
Read the value from the $_ENV auto-global array as shown in Example 8-33.
Reading an environment variable
Environment variables are named values associated with a process. For instance, in Unix, you can check the value of $_ENV['HOME'] to find the home directory of a user, as shown in Example 8-37.
Reading another environment variable
Early versions of PHP automatically created PHP variables for all environment variables by default. As of 4.1.0, php.ini-recommended disables this because of speed considerations; however, php.ini-dist continues to enable $_ENV for backward compatibility.
The $_ENV array is created only if the value of the variables_order configuration directive contains E. If $_ENV isn't available, use getenv( ) to retrieve an environment variable, as shown in Example 8-35.
Using getenv( )
The getenv( ) function isn't available if you're running PHP as an ISAPI module.
8.14.4. See Also
Recipe 8.15 on setting environment variables; documentation on getenv( ) at http://www.php.net/getenv; information on environment variables in PHP at http://www.php.net/reserved.variables.php#reserved.variables.environment.