Reads a variable

```ebnf
variable ::= &? ...complex @todo
```

Some samples of parsed code :
```php
&$var                      // simple var
$var                      // simple var
classname::CONST_NAME     // dynamic class name with const retrieval
foo()                     // function call
$var->func()->property    // chained calls
```
function(read_only, encapsed, byref) {
    let result;

    // check the byref flag
    if (!byref && this.token === "&") {
      byref = true;
      this.next();
    }

    // reads the entry point
    if (this.is([this.tok.T_VARIABLE, "$"])) {
      result = this.read_reference_variable(encapsed, byref);
    } else if (
      this.is([
        this.tok.T_NS_SEPARATOR,
        this.tok.T_STRING,
        this.tok.T_NAMESPACE
      ])
    ) {
      result = this.node();
      const name = this.read_namespace_name();
      if (
        this.token != this.tok.T_DOUBLE_COLON &&
        this.token != "(" &&
        ["parentreference", "selfreference"].indexOf(name.kind) === -1
      ) {
        // @see parser.js line 130 : resolves a conflict with scalar
        const literal = name.name.toLowerCase();
        if (literal === "true") {
          result = name.destroy(result("boolean", true, name.name));
        } else if (literal === "false") {
          result = name.destroy(result("boolean", false, name.name));
        } else {
          // @todo null keyword ?
          result = result("identifier", name);
        }
      } else {
        // @fixme possible #193 bug
        result.destroy(name);
        result = name;
      }
    } else if (this.token === this.tok.T_STATIC) {
      result = this.node("staticreference");
      const raw = this.text();
      this.next();
      result = result(raw);
    } else {
      this.expect("VARIABLE");
    }

    // static mode
    if (this.token === this.tok.T_DOUBLE_COLON) {
      result = this.read_static_getter(result, encapsed);
    }

    return this.recursive_variable_chain_scan(result, read_only, encapsed);
  }