PHP is a cross-platform scripting language
intended specifically for the Web.
PHP was thought to be faster than Perl, but it is less powerful and
flexible (no modules, no namespaces, objects proper only in PHP 5,
no closures, limited anonymous functions capability), and sports an easy to
learn core language.
(A recent
study by Yahoo, however, claimed mod_perl had better performance than
mod_php.)
PHP, as a language, has several potential security issues, which causes
sloppy code to become vulnerable more easily.
Secure code can still be written, but it takes more discipline.
The PHP back-end itself has had a relatively poor security record, with many
vulnerabilities discovered in it.
PHP has many configuration options, which affect its run-time behaviour.
This makes writing code that can be deployed on different systems with
different configurations more difficult.