Programming PHP

not much point in using this technique, as we get the array values from files. But if
the XML document or XSL transformation is dynamically generated, fetched from a
database, or downloaded over a network connection, it’s more convenient to process
from a string than from a file.
Example 11-14. XSL transformation from variables
$xml, '/_xsl' => $xsl);
$processor = xslt_create( );
$result = xslt_process($processor, 'arg:/_xml', 'arg:/_xsl', NULL, $arguments);
if(!$result) exho xlst_error($processor);
xslt_free($processor);
echo "$result";
?>
Although it doesn’t specifically discuss PHP, Doug Tidwell’s XSLT (O’Reilly) provides
a detailed guide to the syntax of XSLT stylesheets.
Web Services
Historically, every time there’s been a need for two systems to communicate, a new
protocol has been created (for example, SMTP for sending mail, POP3 for receiving
mail, and the numerous protocols that database clients and servers use). The idea of
web services is to remove the need to create new protocols by providing a standardized
mechanism for remote procedure calls, based on XML and HTTP.
Web services make it easy to integrate heterogeneous systems. Say you’re writing a
web interface to a library system that already exists. It has a complex system of database
tables, and lots of business logic embedded in the program code that manipulates
those tables. And it’s written in C++. You could reimplement the business logic
in PHP, writing a lot of code to manipulate tables in the correct way, or you could
write a little code in C++ to expose the library operations (e.g., check out a book to
this user, see when this book is due back, see what the overdue fines are for this user)
as a web service. Now your PHP code simply has to handle the web frontend; it can
use the library service to do all the heavy lifting.
XML-RPC and SOAP are two of the standard protocols used to create web services.
XML-RPC is the older (and simpler) of the two, while SOAP is newer and more complex.
Microsoft’s .NETinitiative is based around SOAP, while many of the popular
web journal packages, such as Frontier and blogger, offer XML-RPC interfaces.
PHP provides access to both SOAP and XML-RPC through the xmlrpc extension,
which is based on the xmlrpc-epi project (see http://xmlrpc-epi.sourceforge.net for
280 | Chapter 11: XML
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