Unless you've just come back from an extended journey to Alpha Centauri, you may have heard of the World Wide Web (WWW). The WWW is the new communication media of the 20th and 21st centuries, and throughout the Web you cannot help but hear the word, "Dynamic".
The aggressive momentum as been attributed to several key factors; ease of implementation and integration with the Apache Webserver (which serves more web pages than any other web server made). PHP is also a simple language to learn, and does not have a lot of overhead. For example, to progam the classic "Hello World" example in PHP, you would simply type:
<SCRIPT LANGUAGE=PHP> print "Hello World!\n"; </SCRIPT>