too long

I have a multiple=multiple drop down list and I want to check if a user has selected any option or not using PHP code. If not, then I want to display an error message. How can I do that?

<select name="ddlskill" multiple="multiple">
    <option value="Cake PHP">Cake PHP</option>
    <option value="Database">Database</option>
    <option value="hello">hello</option>
    <option value="Java">Java</option>
    <option value="Jquery">Jquery</option>
    <option value="JSON">JSON</option>
    <option value="JSP">JSP</option>
    <option value="Mysql">Mysql</option>
    <option value="PHP">PHP</option>
    <option value="xyz">xyz</option>
    <option value="XYZ2">XYZ2</option>                
</select>

You should declare your html like this :

<select name="ddlskill[]" multiple="multiple">
    <option value="Cake PHP">Cake PHP</option>
    <option value="Database">Database</option>
    <option value="hello">hello</option>
    <option value="Java">Java</option>
    <option value="Jquery">Jquery</option>
    <option value="JSON">JSON</option>
    <option value="JSP">JSP</option>
    <option value="Mysql">Mysql</option>
    <option value="PHP">PHP</option>
    <option value="xyz">xyz</option>
    <option value="XYZ2">XYZ2</option>   
</select>

And then in PHP, you'll get an array of values :

if( !empty($_POST['ddlskill']) ){
    print_r($_POST['ddlskill']);
    // [PHP, JSP, Java, ...]
}

Maybe helpful,

  <?php 
    if( isset($_POST) && !empty($_POST['ddlskill'])){
    echo "<pre>";print_r($_POST['ddlskill']);
        // [PHP, JSP, Java, ...]
    }
    ?>

<form action="<?php echo htmlentities($_SERVER['PHP_SELF']);?>" method="post">
<select name="ddlskill[]" multiple="multiple">
    <option value="Cake PHP">Cake PHP</option>
    <option value="Database">Database</option>
    <option value="hello">hello</option>
    <option value="Java">Java</option>
    <option value="Jquery">Jquery</option>
    <option value="JSON">JSON</option>
    <option value="JSP">JSP</option>
    <option value="Mysql">Mysql</option>
    <option value="PHP">PHP</option>
    <option value="xyz">xyz</option>
    <option value="XYZ2">XYZ2</option>   
</select>
<button type="submit"> Submit </button>
</form>

  <?php 
    if(!empty($_POST['submit'])){
      if(is_array($_POST['ddlskill']))
      {
       foreach($_POST['ddlskill'] as $key=>$value)
       {
       print_r($value);
       }
      }
      else
      {
       print_r($_POST['ddlskill']);
      }
    }
    ?>

<form action="<?php echo htmlentities($_SERVER['PHP_SELF']);?>" method="post">
<select name="ddlskill[]" multiple="multiple">
    <option value="Cake PHP">Cake PHP</option>
    <option value="Database">Database</option>
    <option value="hello">hello</option>
    <option value="Java">Java</option>
    <option value="Jquery">Jquery</option>
    <option value="JSON">JSON</option>
    <option value="JSP">JSP</option>
    <option value="Mysql">Mysql</option>
    <option value="PHP">PHP</option>
    <option value="xyz">xyz</option>
    <option value="XYZ2">XYZ2</option>   
</select>
<button type="submit" name="submit"> Submit </button>
</form>