PHP is not one that comes up often, there is not a significant overlap between PHP developers and ColdFusion developers. But, in the interests of presenting the complete story, here is what you need to know.
PHP is also script based, pages contain code which are processed by the PHP engine. The PHP engine itself is open source, and the PHP language uses a syntax borrowed from C, Java, and Perl (the latter is important, as PHP is particularly popular with former Perl developers). PHP runs on all sorts of systems, and uses functions and API for all sorts of processing.
PHP’s stated goal (as per the official PHP FAQ) is to “allow web developers to write dynamically generated pages quickly”. Ironically, developers with both CFML and PHP experience will tell you that while PHP development may be quicker than using C or Java, it does not come close to that of ColdFusion and CFML.
There is no refuting PHP’s power, PHP developers have an impressive and rich set of functions and language extensions available, and that is definitely part of PHP’s appeal. But this power comes at a cost, it takes a lot more PHP code to do what simple CFML tags do, and as explained before, more code = greater cost. If you truly need that power, than PHP may indeed be an option worth considering (although CFML+Java would likely give you the same power and more). But if what you really need is to “write dynamically generated pages quickly”, then PHP leaves much to be desired.
One of the most compelling arguments for PHP is its cost, there is none. But the cost of software is only one part of the total cost of application development, arguably the least significant coast in the long run. It is for this reason that despite being incredibly popular with developers and consulting shops, corporate users have been slow to adopt PHP as a development platform (which in turn is why the PHP versus ColdFusion discussion comes up so infrequently).
The bottom line is if you really need the power and flexibility of PHP, and can justify the longer development cycles and more code to manage and maintain, then PHP should seriously be considered. But if time to completion and total cost of ownership are an issue, then ColdFusion wins hands down.