Question on-topicness questionable, but...
The most logical reason why PHP is unsuited for neural networks is that PHP is, well, intended to be used for server side webpages. It can connect to various external resources, such as databases, via native language features. It is very much a glue language, and not a processing language. PHP is also mostly stateless, only allowing you to store state in either clients, file storage or databases.
As such, it's not suitable for this sort of thing - not because PHP is a high level language, but rather because it's so request based and focused towards creating pages to serve to clients.
That won't stop people from trying, though - there are various esoteric programming languages out there in which regular programming would be an insane task or not possible at all - but from a ease of development perspective, making a neural network in PHP makes no sense.