Aquí lo tienes espero que te sirva:
Código PHP:
<form name="formulario1" method="POST" action="--WEBBOT-SELF--"><p>
<script>function resaltar(q){
for(i=0;i<document.forms['formulario1']['select1'].length;i++)if(document.forms[0][1][i].value.indexOf(q) == 0){
document.forms[0][1].selectedIndex = i;
i = document.forms[0][1].length;
}
}</script>
<input type="text" name="T1" size="20" onkeyup="resaltar(this.value)"><br>
<select name="select1" size="1" name="D1">
<option value="a">a</option>
<option value="as">as</option>
<option value="asf">asf</option>
<option value="asfr">asfr</option>
<option value="asfrt">asfrt</option>
</select></p>
</form>