Extending and Embedding PHP

Sebastian Bergmann writes "PHP is a widely-used general-purpose scripting language that is especially suited for Web development. The interpreter that executes programs written in the PHP programming language has been designed from the ground up to be easily embeddable (for instance into the Apache Web Server) and extendable. This extensibility is one of the reasons why PHP became the favourite "glue" of the Web: functionality from existing third-party libraries (database clients or image manipulation toolkits, for instance) can be made available through PHP with the ease of use you expect from a scripting language." Read the rest of Sebastian's review.

Extending and Embedding PHP
author	Sara Golemon
pages	448
publisher	SAMS
rating	9
reviewer	Sebastian Bergmann
ISBN	067232704X
summary	The new standard work on extending and embedding PHP.

"Extending and Embedding PHP" by Sara Golemon, a long-time contributor to the PHP project, remedies the fact that the internals of PHP are far from being as well documented as the rest of PHP. It brings writing extensions for PHP "to the masses", so to speak.

After a short introduction that makes the reader familiar with terms like PHP Extension, Zend Extension, Userland, and Internals that are used throughout the book, Chapter 1 ("The PHP Life Cycle") opens with an overview of how the PHP Interpreter works and what parts (TSRM, Zend Engine, SAPI, "PHP") it comprises.

Chapter 2 ("Variables from the Inside Out") shows how PHP handles and stores variables internally. The reader learns how to distinguish types, set and retrieve values, as well as how to work with symbol tables. It is in this chapter that the fundamental unit of data storage in PHP, the so-called zval (short for Zend Value) is discussed.

Chapter 3 ("Memory Management") builds upon the previous chapter and discusses more advanced operations on zvals, for instance creating and dealing with copies of a zval or the destruction of a zval when it is no longer needed. To this extent, the Zend Memory Manager is discussed as well as underlying principles such as Reference Counting and Copy-on-Write, for instance.

Chapter 4 ("Setting Up a Build Environment") guides the reader through setting up an environment, either on *NIX or on Microsoft Windows, for the development and debugging of PHP and PHP extensions.

After these first four chapters, the reader is ready to go about writing his or her first PHP extension. Chapter 5 ("Your First Extension") takes the reader through the steps necessary to write and build a simple working PHP extension. The following chapters build upon the knowledge gained here, so that the reader can ultimately implement or change any type of PHP feature.

Chapter 6 ("Returning Values") explains how to pass values (by value, by reference, and through their parameter stack using references) from internal (C-level) functions or methods to userland (PHP-level).

Chapter 7 ("Accepting Parameters") deals with the mechanisms involved in accepting parameters from userland calls to an internal function or method. This includes the discussion of the zend_parse_parameters() API which makes the parameters that are passed to the internal function or method as indirect zval references usable in your C-code. The handling of optional and arbitrary numbers of parameters is explained as well as the usage of type hinting and its arg_info API.

Chapter 8 ("Working with Arrays and Hash Tables") explains the Zend Engine's HashTable API, which is used to store any piece of data of any size, in detail. Its different data storage mechanisms supported are introduced and compared. To quote from the book, "A HashTable is a specialized form of a doubly linked list that adds the speed and efficiency of vectors in the form of lookup indices". Since these structures are used heavily throughout the Zend Engine and PHP and its extensions, a good understanding of this API is vital for any aspiring PHP extension developer.

Chapter 9 ("The Resource Data Type") introduces the reader to the first complex data type (excluding the Array data type that was discussed in the previous chapter, which is just a collection containing primitive data types like strings or numbers). A resource can be, for instance, a connection to a database. It allows the PHP extension developer to "connect abstract concepts like opaque pointers from third-party libraries to the easy-to-use userspace scripting language that makes PHP so powerful".

Chapters 10 ("PHP 4 Objects") and Chapter 11 ("PHP 5 Objects") delve into the last data type supported by the Zend Engine: objects. Sara Golemon dedicates one chapter each to the respective APIs of PHP 4 and PHP 5 because of the huge advancements that were introduced in PHP 5 and that totally changed the APIs.

After the previous chapter, all data types supported by the Zend Engine have been discussed and the book revisits a topic discussed earlier in the book: that of the PHP Interpreter's life cycle. Chapter 12 ("Startup, Shutdown, and a Few Places in Between") explains how to add state to a PHP extension by using thread-safe globals. Along the way, concepts such as internal and external (super) globals as well as thread safety are discussed.

Chapter 13 ("INI Settings") shows how a PHP extension can be made ready for runtime configuration through php.ini settings.

The next three chapters ("Accessing Streams", "Implementing Streams", and "Diverting the Stream") make the reader familiar with yet another important API of PHP: the Streams API. All file input/output in PHP userspace is processed through PHP's Streams Layer. This layer, that was introduced in PHP 4.3, is what makes working with files, compressed files, and remote files, for instance, seamlessly in PHP. The reader learns how to work with streams as well as how to expose streamable resources, whether remote network input/output or local data sources, using the Streams API, thus avoiding the need to reimplement all the tedious bits and pieces that are normally associated with this.

Chapter 17 ("Configuration and Linking") builds upon the tools and techniques introduced in Chapter 4 and adds the GNU autotools (autoconf, automake, and friends) to the reader's set of tools. These tools, if used correctly, allow the extension to be built in environments that the extension author does not know or has no access to.

Chapter 18 ("Extension Generators") takes a look at ext_skel (which comes with the source distribution of PHP) and PECL_Gen (which can be obtained, as the name suggests, from PECL, the PHP Extension Community Library). These two tools automate most of the steps described in the previous chapter and take a lot of tedious work out of the extension writer's hands.

Starting with simple embedding examples, the reader learns in Chapter 19 ("Setting Up a Host Environment") and Chapter 20 ("Advanced Embedding") how the PHP Interpreter can be embedded into almost any other application.

The book concludes with the "Zend API Reference", "PHP API Reference", "Extending and Embedding Cookbook", and "Additional Resources" appendixes. The first two are a great resource for both novice and experienced PHP extension writers (even for people working on PHP and the Zend Engine itself). The third features a collection of common use code snippets while the last one points the reader into the direction of PECL by suggesting a couple of existing extensions to look at and learn from.

Since the topic of this book is to extend the PHP Interpreter using extensions written in the C programming language (or to embed it into an application that is written in C), a good understanding of C syntax, its datatypes, and pointer management is important to get the most out of this book.

Being a contributor to the PHP project for about six years now, I have been looking forward to this book. True, there is always the source code of the PHP Interpreter as a source of information on how "things work". But although being the ultimate reference, reading the source code cannot replace a thoughtfully structured and well written guide that gets you started. If you are looking for such a guide, look no further: you will find it in this excellent book.

Although it deals with a very technical topic, "Extending and Embedding PHP" is readable and the many code examples are easy to follow. The reader profits from the knowledge of the author, who has been involved in the PHP project as a core developer for over four years now and is also the author and maintainer of a dozen PHP extensions that are available through PECL. The book covers both major versions of PHP that are currently used, PHP 4 and PHP 5, and it will continue to serve its purpose when PHP 6 comes out next year.

Sebastian Bergmann spends his free time with the development of Free Software, is a member of the PHP and Gentoo Linux development teams and author of a variety of PHP software projects such as PHPUnit."

---

You can purchase Extending and Embedding PHP from bn.com. Slashdot welcomes readers' book reviews -- to see your own review here, read the book review guidelines, then visit the submission page.

php-embed

[SIGALRM]

The book sounds interesting. There's also an often-overlooked capability of PHP: the ability to use php-embed [php.net] to run embedded PHP within a C/C++ app. For example, our company created an HL7 accelerator--we chose PHP as the embedded language in our product--by which users can more easily create custom data transformations.

The reason? PHP is easy to use, loosely-typed (which happened to be an advantage in this case), fast, and of course the license works. It was a great decision.

PHP-embed is basically just a TSRMLS function wrapper. It's pretty straightforward; for example, zval integration is easy as pie, as I recall, something like:

zval *zarray;
MAKE_STD_ZVAL(zarray);
...

if ( array_init(zarray) == FAILURE ) {
// ... something wrong
}

add_assoc_string(zarray, str_name, str_val, 1);

ZEND_SET_SYMBOL(&EG(symtab), tokenlevel, zarray);

Save $18.50 by buying the book here!

[Anonymous Coward]

Save yourself $18.50 by buying the book here: Extending and Embedding PHP [amazon.com]. And if you use the "secret" A9.com discount [amazon.com], you can save an extra 1.57%! That's a total savings of $18.99, or 38.58%!

Re:5 of first 7 comments trolling

[self assembled struc]

(It's a SQL injection waiting to happen, what if I load this page with u="0;drop table users;"?)

Well, one would hope that the READ access you've given to the user you're connecting to the SQL DB with doesn't have the ability to run DROP TABLE.

It's about access control too.

I mean I agree with you on the fact that SQL injection attacks are easier in PHP, but lets face it, you can also design any language to be vunerable. PHP just makes it a little easier by not requiring some sort of connection library (and, in fact, not promoting one either)

Re:5 of first 7 comments trolling

[KidSock]

For those who may be curious, the proper way to actually prevent SQL injections is to wrap anything coming in with a function that calls stripslashes() and mysql_escape_string() (or equivalent function for another db). For example, the function I use looks like the following (this also adds quotes around anything that is not numeric):

[sorry for the poor formatting, ./ is highly broken when it comes to posting code] // Quote variable to make safe
function quote_smart($value)
{ // Stripslashes
        if (get_magic_quotes_gpc()) {
                $value = stripslashes($value);
        } // Quote if not integer
        if (!is_numeric($value)) {
                $value = "'" . mysql_escape_string($value) . "'";
        }

        return $value;
}

Now you call this through sprintf like:

$res=mysql_query(sprintf("select data from users where userid=%s", quote_smart($_GET['u']));

Now this is perfectly safe from SQL injection. Anyone who has done real web programming knows all about this and knows that you need to deal with this sort of thing regardless of what language you're using.

Also, whenever you emit data that will appear in HTML you also need to wrap it. This time you just use the builtin htmlentities() function like:

echo "<input name=\"u\" type=\"text\" value=\"" . htmlentities($user) . "\">\n";

This prevents cross site scripting. Again, no different from any other language.

PS: IMHO if someone goes out of their way to claim something "sucks" they probably don't know what they're talking about. Try the other languages and read the documentation so that you can evaluate which is best for your project.