Thank's alot bro... I tried your code but it didn't work. Here is a test:
Code:
<script>
function tttt(){
	var myList = document.getElementById("lstname");
  	for(i=0;i<myList.length;i++){
    	this.myList[i].selected = true;
  	}
}
</script>
<?php
	if(isset($_POST['sbbutton'])){
		foreach ($_POST['lstname'] as &$value) {
    		echo $value."<BR>";
		}
	}
?>
<form name='myform' method='POST' onsubmit='tttt()' action='<?php $_SERVER[PHP_SELF] ?>'>
<select name='lstname[]' multiple size='8'>
<option name='t1'>test1</option>
<option name='t2'>test2</option>
</select>	
<input type='submit' name='sbbutton' value='Submit'>
</form>