If you are asking me for help to compile PHP, I don't know much about your environment, so this may not be accurate. You would need access to the server for this. If your service is hosted your ISP will have to do this.
Go to www.php.net and download the latest version - 4.3.10. Untar it so you get the source directory.
go into that directory - I would read the INSTALL file before proceeding, but here is the gist of it:
./configure --with-apxs --with-mysql --with-xml
then, when that is done if it is a success, do:
That will build PHP (assuming for Apache, with MySQL and XML - if you want other stuff you need to read the docs) and install it in all the right places.
Then you need to shutdown and restart Apache (service httpd restart) and you should be good to go.
If you're not comfortable doing this, don't. You could make things worse.
Hope this helps.