ASP is not really a language in itself, it's an acronym for Active Server Pages, the actual language used to program ASP with is Visual Basic Script or JScript. The biggest drawback of ASP is that it's a proprietary system that is natively used only on Microsoft Internet Information Server (IIS). This limits it's availability to Win32 based servers. There are a couple of projects in the works that allows ASP to run in other environments and webservers: InstantASP from Halcyon (commercial), Chili!Soft ASP from Chili!Soft (commercial). ASP is said to be a slower and more cumbersome language than PHP, less stable as well. Some of the pros of ASP is that since it primarily uses VBScript it's relatively easy to pick up the language if you're already know how to program in Visual Basic. ASP support is also enabled by default in the IIS server making it easy to get up and running. The components built in ASP are really limited, so if you need to use "advanced" features like interacting with FTP servers, you need to buy additional components.

Yes, the server-side asp2php is the one most often referred to as well as this client-side option.

PHP is commonly said to be faster and more efficient for complex programming tasks and trying out new ideas. PHP is generally referred to as more stable and less resource intensive as well. Cold Fusion has better error handling, database abstraction and date parsing although database abstraction is addressed in PHP 4. Another thing that is listed as one of Cold Fusion's strengths is its excellent search engine, but it has been mentioned that a search engine is not something that should be included in a web scripting language. PHP runs on almost every platform there is; Cold Fusion is only available on Win32, Solaris, Linux and HP/UX. Cold Fusion has a good IDE and is generally easier to get started with, whereas PHP initially requires more programming knowledge. Cold Fusion is designed with non-programmers in mind, while PHP is focused on programmers.

A great summary by Michael J Sheldon on this topic has been posted to the PHP mailing list. A copy can be found at http://marc.theaimsgroup.com/?l=php-general&m=95602167412542&w=1.

The biggest advantage of PHP over Perl is that PHP was designed for scripting for the web where Perl was designed to do a lot more and can because of this get very complicated. The flexibility / complexity of Perl makes it easier to write code that another author / coder has a hard time reading. PHP has a less confusing and stricter format without losing flexibility. PHP is easier to integrate into existing HTML than Perl. PHP has pretty much all the 'good' functionality of Perl: constructs, syntax and so on, without making it as complicated as Perl can be. Perl is a very tried and true language, it's been around since the late eighties, but PHP is maturing very quickly.