PHP is a server side scripting language that is embedded in HTML. It is used to manage dynamic content, databases, session tracking, even build entire e-commerce sites. It is integrated with a number of popular databases, including MySQL, PostgreSQL, Oracle, Sybase, Informix, and Microsoft SQL Server. ...

It is a very good habit to enter comment in your code. Entering comment in code helps you to keep track of the area and later you can easily debug them (if required). The browser ignores comments in HTML.

Variables begin with a dollar sign ($) and are followed by a concise, meaningful name. The variable cannot begin with a numeric character.

Sometimes it is convenient to be able to have variable variable names. That is, a variable name which can be set and used dynamically.

Variables in PHP are represented by a dollar sign followed by the name of the variable. PHP supports the basic data types of strings, integers, double precision floating point numbers, etc. Variables are not tied to a specific data type, and may take any of the data types.

PHP transparently supports HTTP cookies. Cookies are a mechanism for storing data in the remote browser and thus tracking or identifying return users. You can set cookies using the setcookie() or setrawcookie() function. Cookies are part of the HTTP header, so setcookie() must be called before any output is sent to the browser.

Session support in PHP consists of a way to preserve certain data across subsequent accesses. This enables you to build more customized applications and increase the appeal of your web site.
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Source: home.arcor.de