Now that PHP has grown to be a popular scripting language, there are
a lot of public repositories and libraries containing code you can reuse.
The PHP developers have largely tried to preserve backwards compatibility,
so a script written for an older version will run (ideally) without changes
in a newer version of PHP. In practice, some changes will usually be needed.
Two of the most important recent changes that affect old code are:
The old $HTTP_*_VARS arrays are not available as of
PHP 5.4.0. The following
were introduced in PHP » 4.1.0.
They are: $_GET, $_POST,
$_REQUEST, and $_SESSION.
External variables are no longer registered in the global scope by
default. In other words, as of PHP
» 4.2.0 the PHP directive
off by default in php.ini. The preferred
method of accessing these values is via the superglobal arrays mentioned
above. Older scripts, books, and tutorials may rely on this
directive being on. If it were on,
for example, one could use $id from the URL
http://www.example.com/foo.php?id=42. Whether on
or off, $_GET['id'] is available.
For more details on these changes, see the section on
and links therein.