it is the problem, i can't undertand anyway. i have the following simple script

    <input class="input"  type="text" name="l_username" style="color: #ccc;" 
     value= " <?if ($_POST[l_username] != '') 
              echo $_POST[l_username];
              else echo 'something';?>"   
     onfocus="if (this.value == 'something') {
 this.value='';this.style.color='black';}" />

onfocus doesn't work here, but when i delete php script from value, it works

<input class="input"  type="text" name="l_username" style="color: #ccc;" 
         value= " something"   
         onfocus="if (this.value == 'something') {
     this.value='';this.style.color='black';}" />

it works fine. could you tell me why? thanks