原始的解释器模式(Interpreter Pattern)

<?php  
  
interface MathExpression  
{  
    public function evaluate( array $values );  
}  
  
/** 
 * A terminal expression which is a literal value. 
 */  
class Literal implements MathExpression  
{  
    private $_value;  
  
    public function __construct( $value ) {  
        $this->_value = $value;  
    }  
  
    public function evaluate( array $values ) {  
        return $this->_value;  
    }  
}  
  
/** 
 * A terminal expression which represents a variable. 
 */  
class Variable implements MathExpression  
{  
    private $_letter;  
  
    public function __construct( $letter ) {  
        $this->_letter = $letter;  
    }  
  
    public function evaluate( array $values ) {  
        return $values[$this->_letter];  
    }  
}  
  
/** 
 * Nonterminal expression. 
 */  
class Sum implements MathExpression  
{  
    private $_a;  
    private $_b;  
  
    public function __construct( MathExpression $a, MathExpression $b ) {  
        $this->_a = $a;  
        $this->_b = $b;  
    }  
  
    public function evaluate( array $values ) {  
        return $this->_a->evaluate( $values ) + $this->_b->evaluate( $values );  
    }  
}  
  
/** 
 * Nonterminal expression. 
 */  
class Multiply implements MathExpression  
{  
    private $_a;  
    private $_b;  
  
    public function __construct( MathExpression $a, MathExpression $b ) {  
        $this->_a = $a;  
        $this->_b = $b;  
    }  
  
    public function evaluate( array $values ) {  
        return $this->_a->evaluate( $values ) * $this->_b->evaluate( $values );  
    }  
}  
  
// 10(a + 3)  
$expression = new Multiply( new Literal( 10 ), new Sum( new Variable( ‘a‘ ), new Literal( 3 ) ) );  
echo $expression->evaluate( array( ‘a‘ => 111 ) ), "\n";  
  
运行结果：1140  
[Finished in 0.1s]