I have been learning PHP, mainly to study the WordPress source code. I have never had a good impression of PHP but I know it is used very widely. So when I found relevant comments from people qualified to comment I was surprised how clearly they say that PHP has problems.
First, see Announcing a specification for PHP « HHVM posted on July 30, 2014 that says
the PHP language has always been missing a language specification and announces the development of one.
The following is based on What factors during the development of PHP contributed to it being such a poorly designed language? - Quora.
PHP was originally written by Rasmus Lerdorff. He does not want to be responsible for the design of the language. There is not anyone specifically that is. There is no official mechanism for deciding what the design of the language should be. The result is a poorly designed language.
Orinally, PHP was a templating markup but not a language. The original designer of PHP does not consider PHP to be a language.
What are the horrors of PHP? has many useful comments but I won't try to summarize them here. The following were referenced from there. I have not read the following.