[php-langspec][Issue #248] - primary-expression and assignment-expression not being used

You can view the Issue on github:
https://github.com/php/php-langspec/issues/248

Opened By: dende
Issue Description:
It might be, that I'm missing something, because formal language definition is somewhat new to me, but right now it seems to me, that a [statement](https://github.com/php/php-langspec/blob/master/spec/19-grammar.md#grammar-statement)  can never generate a simple expression like `$v = 1`.

A [statement](https://github.com/php/php-langspec/blob/master/spec/19-grammar.md#grammar-statement) can generate an [expression statement](https://github.com/php/php-langspec/blob/master/spec/19-grammar.md#grammar-expression-statement), which can generate an [expression](https://github.com/php/php-langspec/blob/master/spec/19-grammar.md#grammar-expression).

<pre>
<i id="grammar-expression-statement">expression-statement:</i>
   <i><a href="#grammar-expression">expression</a></i><sub>opt</sub>   ;
</pre>


But I can't find any way to generate a [primary-expression](https://github.com/php/php-langspec/blob/master/spec/19-grammar.md#grammar-primary-expression) or an [assignment-expression](https://github.com/php/php-langspec/blob/master/spec/19-grammar.md#grammar-assignment-expression) out of an [expression](https://github.com/php/php-langspec/blob/master/spec/19-grammar.md#grammar-expression). When searching the repository for the terms, I can only find their definitions, not any usages of them: [repository search for 'primary-expression'](https://github.com/php/php-langspec/search?q=primary-expression)

What am I missing here?