PHP is not outdated, it's simply victim of an outdated methodology; not making enough of an effort to reduce complexity. Complexity is reduced by things like namespaces, polymorphism, consistent syntax, and simplified debugging.

PHP was originally designed to have a short learning curve for non-programmers. Retrospectively, we see that many bad design decisions were made a long time ago. PHP became popular because it was so easy to use, but since it didn't have a community of experts, PHP lacked a good developer ecosystem for a long time.

We see large companies like Facebook using PHP is because it was a great and powerful language to use, compared to the alternatives at the time. Once you have a large, quickly growing company, rewriting your application in a different language is very difficult. Your whole staff needs to learn the new language, and thousands of hours need to be devoted to porting code instead of maintenance and development.

PHP is one of the most popular languages and it's in very high demand. You shouldn't consider that proof that it's the best language, though. There are a number of reasons why modern languages aren't the most popular. They haven't been around for as long, there might not be a good ecosystem yet, and it takes a while for the average programmer to update their skills.

Here is a classic overview of the problems with PHP: PHP: a fractal of bad design

All of that said, in recent years PHP has improved immensely. There is now better support for OO programming, namespaces, and debugging. Most importantly, more libraries and frameworks have been created that abstract away the icky bits of PHP and allow programmers to make things that are on par with other, newer languages. The new PHP - Programming - O'Reilly Media