PHP is designed - where necessary to the exclusion of other uses - as a web page tool. In other words, where a language design decision needs to made between something that is good for the web use, but bad for more general use, the decision will always be in favour of the web. "I don't care how it effects the other users" said Rasmus Lerdorf when I attended a lecture of his a year or two back - but what Rasmus (the PHP originator and mentor) means is that he really does care that it should be great for web users.

So ... most of the other language I teach had a "fork" function or equivalent from the early days which lets you split the process in two, with each half performing a different task. In PHP, it's there too - as from 4.1 - under the name pcntl_fork. But forking a complete process can be resource-heavy and tricky to code and there are other options too - such as setting off parallel actions on pipes.

Here's some sample code links.

Pinging multiple hosts in parallel
Grabbing titles from multiple pages in parallel
You can try the second example here if you wish.

You'll notice that I've used the full path to PHP and my test script in the title grabber. That's because the PHP script will be called up with goodness only knows what path and current directory, and it's sometimes needed to get the thing to work. You may also find that some of the php.ini settings hamper you.

Finally, a note of caution. If you write a script that goes out and grabs lots of information from other servers, you're writing an automata or robotic process and you should check that this is acceptable to your target site - their robots.txt file.
(written 2007-01-03)

 H308 - PHP - Searches, and search engines
  [3163] Twitter - the special use of @ # and http: in tweets - (2011-02-09)
  [3159] Returning multiple values from a function call in various languages - a comparison - (2011-02-06)
  [2631] How to show a large result set page by page in PHP - (2010-02-11)
  [2166] Crossrefering documents with uniqueness and inconsistency issues - PHP proof of concept demo - (2009-05-10)
  [2137] Reaching the right people with your web site - (2009-04-23)
  [1735] Finding words and work boundaries (MySQL, Perl, PHP) - (2008-08-03)

H999 - Additional PHP Material
  [3210] Catchable fatal error in PHP ... How to catch, and alternative solutions such as JSON - (2011-03-22)
  [3118] Arrays of arrays - or 2D arrays. How to program tables. - (2011-01-02)
  [2684] Exception handling in PHP - (2010-03-18)
  [2215] If nothing, make it nothing. - (2009-06-02)
  [2073] Extra PHP Examples - (2009-03-09)
  [1623] PHP Techniques - a workshop - (2008-04-26)
  [1519] Flipping images on your web page - (2008-01-26)
  [1505] Script to present commonly used images - PHP - (2008-01-13)
  [1485] Copyright and theft of images, bandwidth and members. - (2007-12-26)
  [1451] More PHP sample and demonstration programs - (2007-12-01)
  [1391] Ordnance Survey Grid Reference to Latitude / Longitude - (2007-10-14)
  [1390] Converting from postal address to latitude / longitude - (2007-10-13)
  [1389] Controlling and labelling Google maps via PHP - (2007-10-13)
  [1270] PHP Standalone - keyboard to screen - (2007-07-18)
  [1194] Drawing hands on a clock face - PHP - (2007-05-19)
  [1104] Drawing dynamic graphs in PHP - (2007-03-09)
  [1053] Sorting people by name in PHP - (2007-01-26)
  [1010] Dates, times, clickable diarys in PHP - (2006-12-28)
  [937] Display an image from a MySQL database in a web page via PHP - (2006-11-22)
  [917] Syntax checking in PHP - (2006-11-07)
  [839] Reporting on the 10 largest files or 10 top scores - (2006-08-20)
  [822] PHP - a team member leaves - (2006-08-04)
  [806] Check your user is human. Have him retype a word in a graphic - (2006-07-17)
  [789] Hot answers in PHP - (2006-07-02)
  [687] Presentation, Business and Persistence layers in Perl and PHP - (2006-04-17)
  [665] PHP Image viewing application - (2006-04-01)
  [603] PHP - setting sort order with an associative array - (2006-02-13)
  [563] Merging pictures using PHP and GD - (2006-01-13)
  [493] Running a Perl script within a PHP page - (2005-11-12)
  [483] Double Dollars in PHP - (2005-11-02)
  [468] Stand alone PHP programs - (2005-10-18)
  [372] Time calculation in PHP - (2005-07-08)
  [337] the array returned by preg_match_all - (2005-06-06)
  [322] More maps - (2005-05-23)
  [320] Ordnance Survey - using a 'Get a map' - (2005-05-22)
  [239] What and why for the epoch - (2005-03-08)
  [54] PHP and natural sorting - (2004-09-19)

W603 - Web and Intranet - Server Side Technologies
  [2282] Checking robots.txt from Python - (2009-07-12)
  [2055] Effect on server when memory runs out and swapping starts - (2009-02-26)
  [1749] Using server side and client side programming together - (2008-08-11)
  [1615] PHP training courses every month - (2008-04-18)
  [1554] Online hotel reservations - Melksham, Wiltshire (near Bath) - (2008-02-24)
  [1365] Korn Shell scripts on the web - (2007-09-25)
  [1355] .php or .html extension? Morally Static Pages - (2007-09-17)
  [1031] robots.txt - a clue to hidden pages? - (2007-01-13)
  [732] Where is a web site visitor browsing from - (2006-05-24)
  [653] Easy feed! - (2006-03-21)
  [642] How similar are two words - (2006-03-11)

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PHP - static declaration
Parallel processing in PHP
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Open Source Courses and Business Hotel - products and prices for 2007
css - handling white space and pre
Modernising from tables to cascading style sheets
Search engine placement - long term strategy and success