code: function read_constant_declaration() {
          const result = this.node("constant");
          let constName = null;
          let value = null;
          if (
            this.token === this.tok.T_STRING ||
            (this.php7 && this.is("IDENTIFIER"))
          ) {
            constName = this.node("identifier");
            const name = this.text();
            this.next();
            constName = constName(name);
          } else {
            this.expect("IDENTIFIER");
          }
          if (this.expect("=")) {
            value = this.next().read_expr();
          }
          return result(constName, value);
        }
docs: Reads a constant declaration
  
  ```ebnf
  constant_declaration ::= (T_STRING | IDENTIFIER) '=' expr
  ```
  @return {Constant} [:link:](AST.md#constant)