zval container: all php variables’ value is store in the zval container, which also includes is_ref (boolean value, and default is false, this is indicated whether the variable point to this value is reference variable or not) and refcount (contains how many variables point to this value)

symbol table: all variable name in different scope will be stored in symbol table, which value is pointed to the zval container.

Reference:

• http://www.levijackson.net/php-zvals-and-symbol-tables/
• https://zhuanlan.zhihu.com/p/32720364