What is wrong with PHP 5.0 or 5.1?

PHP 5.0.x had a number of problems with stability and performance. Much of the feature set wasn't fully nailed down yet either, and tools such as SPL and PDO didn't work right until PHP 5.1. Many developers and web hosts shied away from PHP 5 because of it's unfortunately rocky beginnings. Fortunately, PHP 5.0 is not very widely used.

PHP 5.1 was much more stable and reliable, but it also had some issues with performance. It's a viable platform, however, and many projects already require it.

PHP 5 has really come into its own with PHP 5.2, however. It adds a number of important features to the core system that we feel are important to target. Most importantly:

• The Filter extension is a new security component in PHP. Although available as an optional add-on in PHP 5.1, most web hosts did not install it. It is, however, now included by default in PHP 5.2. The filter extension allows developers to read input more safely and securely. That helps make programs more secure, less error prone, and harder to hack.
• Many sites now use Ajax for much of their functionality, and one of the most popular tools for Ajax is the JSON format. JSON is a way of specifying data that both Javascript in the web browser and PHP or other languages on the server can understand, making writing portable Ajax applications easier. PHP 5.2 includes a fast, native mechanism for reading and writing from JSON, just as it does for XML, which makes writing dynamic rich client applications easier.
• Faster, more stable, fewer bugs!