$_ENV -- $HTTP_ENV_VARS [deprecated] — Environment variables
An associative array of variables passed to the current script via the environment method.
These variables are imported into PHP's global namespace from the environment under which the PHP parser is running. Many are provided by the shell under which PHP is running and different systems are likely running different kinds of shells, a definitive list is impossible. Please see your shell's documentation for a list of defined environment variables.
Other environment variables include the CGI variables, placed there regardless of whether PHP is running as a server module or CGI processor.
$HTTP_ENV_VARS contains the same initial information, but is not a superglobal. (Note that
$_ENV are different variables and that PHP handles them as such)
<?php echo 'My username is ' .$_ENV["USER"] . '!'; ?>
This is a 'superglobal', or automatic global, variable. This simply means that it is available in all scopes throughout a script. There is no need to do global $variable; to access it within functions or methods.