使用php如何获取下拉列表中的多个选定值

<form method = &#39;post&#39; action = "demo.php">  
        <h4>选择课程</h4> 
          
        <select name = &#39;subject[]&#39; multiple size = 6>   
            <option value = &#39;英语&#39;>英语</option> 
            <option value = &#39;数学&#39;>数学</option> 
            <option value = &#39;计算机&#39;>计算机</option> 
            <option value = &#39;物理学&#39;>物理学</option> 
            <option value = &#39;化学&#39;>化学</option> 
            <option value = &#39;经济学&#39;>经济学</option> 
        </select> 
        <input type = &#39;submit&#39; name = &#39;submit&#39; value =&#39;提交&#39;> 
</form>

<?php 
      
    // 检查表单是否提交成功
    if(isset($_POST["submit"]))  
    { 
        // 检查是否选中选项
        if(isset($_POST["subject"]))  
        { 
            // 检索每个选中选项
            foreach ($_POST[&#39;subject&#39;] as $subject)  
                print "你选择了 $subject<br/>"; 
        } 
    else
        echo "请先选中一个选项 !!"; 
    } 
?>