9.10.3. Discussion

Because PHP uses the period as a string concatenation operator, a form variable called animal.height is automatically converted to animal_height, which avoids creating an ambiguity for the parser. While $_REQUEST['animal.height'] lacks these ambiguities, for legacy and consistency reasons, this happens regardless of your register_globals settings.

You usually deal with automatic variable name conversion when you process an image used to submit a form. For instance: you have a street map showing the location of your stores, and you want people to click on one for additional information. Here's an example:

<input type="image" name="locations" src="locations.gif">

When a user clicks on the image, the x and y coordinates are submitted as locations.x and locations.y. So, in PHP, to find where a user clicked, you need to check $_REQUEST['locations_x'] and $_REQUEST['locations_y'].

It's possible, through a series of manipulations, to create a variable inside PHP with a period:

${"a.b"} = 123; // forced coercion using {}

$var = "c.d";   // indirect variable naming
$$var = 456;       

print ${"a.b"} . "\n";
print $$var . "\n";
123
456

This is generally frowned on because of the awkward syntax.