I used to be an Eclipse fan, but a few years ago I moved to NetBeans and have never looked back.
I am currently using it for PHP (and MySql/SQlite), C, C++ and JavaScript.
From your request:
free and open source <=== check
FTP support (also SFTP) <=== https://blogs.oracle.com/netbeansphp/entry/ftp_support_added
Code highlight <====== yup
Windows, Mac OS and Linux <=== written in Java, so yup, plus Raspberry Pi et al
PHP, HTML, CSS, Javascript (and a huge amount of other languages) ...
https://netbeans.org/features/ .... "NetBeans IDE offers superior support for C/C++ and PHP developers, providing comprehensive editors and tools for their related frameworks and technologies. In addition, the IDE has editors and tools for XML, HTML, PHP, Groovy, Javadoc, JavaScript, and JSP."
Plus, add your own ... http://wiki.netbeans.org/How_to_create_support_for_a_new_language
You won't ever look for another IDE. Guaranteed, or your money back ;-)