Developing Custom PHP Extensions: Part 1 - What is a PHP extension? (Page 2 of 5 )
A PHP extension, in the most basic of terms, is a set of instructions (i.e. code) that is designed to add functionality to PHP. For example, the widely used GD library (used for the creation of dynamic images) is an extension. This library added new functionality by allowing PHP to generate images on the fly. Another example is the MySQL extension, which allows us to connect to and work with MySQL databases.
What are PHP extensions needed for? There are several reasons why extensions are needed. One of them, as stated above, is to add new functionality to PHP. For instance, where would we be today if someone did not add the functionality to work with MySQL? Where will we be tomorrow if someone does not add the functionality to work with tomorrow’s databases or tomorrow's technologies? As PHP continues to grow, it is likely that new "features" will be required by the ever-growing number of web developers. Some of the new features will be popular enough to be added to the official distribution, while others will not. Either way, those extensions will serve their creators well.
On the other hand, we might use PHP extensions to improve the efficiency and speed of our programs. Some processor intensive functions might be better coded as an extension rather than straight PHP code. Since extensions are written in C (more on the actual coding later), they will work much faster than straight PHP code too.
Another possible reason to employ extensions is to reuse frequently utilized code. Instead of moving the same old functions from project to project, you could place them all in one extension and allow all your projects to utilize that extension.
How do I develop my own extensions? Before this question can be answered, we must look at the different "types" of extensions available. Extensions come in three different flavors: Zend engine extensions, built-in extensions and external extensions.
Zend Engine extensions are extensions that are implemented right into the engine itself. For those of you who do not know, the Zend engine is what PHP is built on. It is the engine that parses, interprets and executes your PHP scripts. Changing the engine itself will change the way PHP works. Anything that will affect the language itself or its features is added to the Zend engine; this includes if statement evaluation, object orientation, mathematical expressions evaluation, etc.
Although extending the engine is possible, it's not recommenced for reasons such as incompatibility with servers that run the officially distributed engine. In other words, not too many server administrators will agree to use an unofficial version of the Zend engine.
Built-in extensions are extensions that are compiled right into PHP and are loaded with the PHP processes. The advantages of this method are: programmers aren't required to load extensions manually, and no extension files are required (since it is compiled right into the PHP binary itself). The disadvantages, on the other hand, are: any changes to the extension will require a complete re-compilation of the PHP binary itself, and the size of the binary will grow with each new extension (as will the amount of memory it will consume).
External extensions are extensions that are manually added during run time. All the functionality of the extension will be available to the script that loaded it. When the script ends, the extension is released and the memory is freed. As you might guess, the advantages are: only the extension itself needs to be re-compiled after any changes and a small PHP binary. Also, you don’t provide the functionality of your extension to scripts that do not require it. And, as always, where advantages go, disadvantages follow: extensions are loaded during run time, a process that takes time, and the programmer must remember to load the extension since it is not automatically available.
Although loading external extensions each time the script is executed takes time, it is fairly quick. I personally do not feel any speed differences when I load my external extensions. Of course, if the site receives heavy traffic, a speed difference might be apparent and built-in extensions might be the most appropriate solution. Nevertheless, in these articles we will develop an external extension. Note that the difference between built-in extensions and external extensions – code wise – is virtually nonexistent.