PHP is a server-side scripting language that is most often used for web development. However it is also used as a general-purpose programming language.
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Pro One of the most common languages
Pro Lots of tutorials online
Pro Most prominent language for web applications
Part of the de facto standard web application stack.
Pro Used by most common CMS platforms
Many clients are looking for an easy-to-update web site that's flexible and free. Drupal and Wordpress fill those needs very well.
Con Poorly designed language
Despite its widespread use, PHP is generally looked upon poorly from a design point of view. The consistency of function names and function argument order, lazily and borderline non-functional implementation of object oriented programming, can only receive requests via POST methods, slow version adoption (the PHP you learn right now may not work on every webserver you'll work on), and a focus on "hacking things together" rather than "doing it right". These are all very common complaints when it comes to working with PHP. While not a bad language to learn, PHP is not at all a good language to learn first, as it will probably teach bad habits.
Con Immense catalog of insecure frameworks
The most serious security problems in websites on the web today are almost universally found in popular PHP frameworks, CMS platforms, libraries and code samples, almost all stemming from poor language design, bad tutorials and awful resources.
Con Most resources are poorly-written
Few resources exemplify the "correct" or secure use of features.
Con Most tutorials are out of date
A lot of very bad tutorials are still widely circulated among beginners, and these tutorials teach very poor programming practices.