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.