why php is more reliable and more use-able language other language dot-net ,java and so on

cause its like this HTML/CSS/PHP/MySQL (they ALL WORK together) and also open source means more people are going to come up with more ideas then you can think of to make your site more dynamic then you need. As well as its allowed to be on MAC/Linux and Windows with out modding a single thing

php is platform independent but others are dependent.

Java is not terribly platform dependant, it'll run on anything that can run the JRE/JVM. That PHP is aimed at web development much more than Java is a more significant difference, but although it is possible you wouldn't try to write a GUI-based program in PHP, it's just not what it's best at. Java can do that much more easily, though it wouldn't be my language of choice (I'm a C++/Qt guy for cross platform application development).

That .NET is platform dependant is true, there are projects that aim to port the framework to other platforms (i.e. Mono), but they're always well behind the actual .NET implementation and are not exactly great alternatives. So .NET only works well on Windows.

More towards the OP - there are lots of frameworks and tools which are fairly well proven and tested available in the PHP ecosystem. By utilising those you can get a bit more reliability than others, but there's nothing inherent to PHP that makes it so, and in fact a lot of PHP-based websites are terribly coded (this is well known, "PHP developer" is almost a dirty phrase) but that's again nothing inherent to the language except it being easy to get started with - and it's usually novice work that is vulnerable or unreliable. An experienced developer can do good work in whatever they need to work with, be it (for websites) PHP, Ruby, Python, whatever.