machines,  generate  and  manipulate  images,  generate  PS  and  PDF  documents,  generate 

and  compress  archive  files,  generate  and  authenticate  OpenSSL  signatures,  provide 

HTTP authentication for access control, handle cookies, query databases and parse XML 

files, to name just a few of its capabilities. 

These three software elements interact as follows: PHP is run as a module in the Apache 

server,  a  background  process  (a   daemon   in  UNIX  or  a   service   in  Windows)  which 

listens for network requests on port 80, the default HTTP port. In practise, this means that 

HTTP  requests  for  all  files  with  the  .php  (or  .php3,  or  .php4  or  .phtml or any arbitrary 

extension nominated in the config file) are run through the PHP engine, usually with the 

intention of outputting hypertext. This is then sent back to Apache and served out to the 

requesting user   hence  hypertext preprocessing . 

PHP  has  extensions  to  interact  with  various  well known  DBMS`s,  including  Oracle, 

PostgreSQL, MSSQL and the above mentioned MySQL. Assuming PHP has been started 

with  the  appropriate  module,  a  script  can  then  connect  to  and  query  any  available 

database on the network (usually localhost, in practice). 

PHP also comes bundled with an event based XML parser extension, which has a number 

of automated functions for parsing XML documents and dropping the results into easily 

accessible  associative  arrays.  An  event based  parser  ignores  DTDs  and  does  not 

automatically take note of the document hierarchy. Instead it treats each occurrence of tag 

and its attributes as an event which triggers some sort of post defined function. 

2.5.2 Client side 

While  the  server side  application  is  focused  on  the  retrival  of  information,  an  equally 

important  part  of  any  piece  of  software  is  the  user  interface,  which  is  expressed  in  the 

languages of the web of HTML and CSS. The Hypertext Markup Language has come a 

long way since the first ever website went live from 

http://nxoc01.cern.ch/

 around the end 

of  1990.  (http://www.w3.org/People/Berners Lee/FAQ.html#Examples).  There  has  been 

an increasing usage of images and other embedded multimedia; the web today is as much 

a graphical environment as a textual one, which is in sharp contrast to the first version of 

HTML,  which  did  not  even  have  an    tag.  Another  issue  has  been  that  of 

competing browser manufacturers often opting to parse HTML in different ways or even 

using  their  own  proprietary  tags  outside  of  the  W3C  specification,  with  the  end  result 

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