PHP is Open Source. In purely practical terms, it means that you are not dependent on a manufacturer to fix things that don't work, nor are you forced to pay for upgrades every year to get a working version. Those of us who have waited for Allaire to get something fixed will appreciate this. There is no IDE or debugger. An IDE is probably not important for most people and a debugger is promised from Zend Tech.
Most people continue using the same environment they use for HTML development.
If you are running on a UNIX or Linux platform, you have the choice between Perl and PHP, both excellent for scripting. Where PHP has the edge is in being much simpler for people new to writing scripts, making it more productive in the earlier stages. Perl also relies on having the correct modules downloaded from CPAN for some of the more off-beat functionality, whereas they come as part of the PHP distribution. On a Windows platform, there is competition from ASP and Cold Fusion. But on a purely technical basis, PHP is as stable on WindowsNT as either of the other two, at least as fast and less resource intensive.