PHP is a widely-used general-purpose scripting language that is especially suited for Web development and can be embedded into HTML. PHP 5 is fast, stable and production-ready and as PHP 6 is on the way, the PHP development team announced that support for PHP 4 will continue until the end of this year only.
After 31 December 2007, there will be no more releases of PHP 4.4, however, they will continue to make critical security fixes available on a case-by-case basis until 8 August 2008. This time frame has been kept to allow people to make their applications suitable to run on PHP 5. The current PHP 5 Stable release is 5.2.3 and historical PHP 4 stable release is 4.4.7 (which will now be discontinued at 2007-12-31).
Thinking about migrating from PHP 4 to PHP 5? PHP 5 and the integrated Zend Engine 2 have greatly improved PHP’s performance and capabilities, but care has been taken to break as little existing code as possible, so migrating your code from PHP 4 to 5 should be easy.
I just checked the WordPress minimum requirements page and to run WordPress your host needs a PHP version 4.2 or greater and MySQL version 4.0 or greater. However, as they had warned developers that WordPress 2.1 will require MySQL 4.0, they will emphasize the new PHP requirements for latest versions of WordPress.