PHP, which stands for "PHP: Hypertext Preprocessor" is a widely-used Open Source general-purpose scripting language that is especially suited for web development and can be embedded into HTML. Its syntax draws upon C, Java, and Perl, and is easy to learn. The main goal of the language is to allow web developers to write dynamically generated web pages quickly, but you can do much more with PHP.
This comprehensive PHP reference will cover all the functions and features of PHP that you should know about.
All these functions are used to manipulate strings in one way or another. Some more specialized sections can be found in the regular expression and URL handling sections. If you are just starting out, maybe you should read this post about string data type. You can also read about all the string functions on the reference page.
All these functions are used to interact with and manipulate arrays in various ways. Arrays can prove very useful when you are storing, managing, and operating on a set of variables. If you are just starting out, maybe you should read this post about arrays in PHP. You can also read about all the array functions on the reference page.
All these functions are used to work with files. You can use them to read or write to a file. You can also rename or delete files with ease. They are part of the PHP core and you don't have to install anything to use these functions. You can also read about all the filesystem functions on the reference page.