Hola lizfranco.
Prueba con esto:
Código PHP:
<html>
<head>
<script language="JavaScript">
function copia(){
with (document.frm){
tex.value = sel.options[sel.selectedIndex].text;
}
}
</script>
</head>
<body>
<form name="frm">
<select name="sel">
<option value="0"></option>
<option value="1">Uno</option>
<option value="2">Dos</option>
<option value="3">Tres</option>
</select><br>
<input type="text" name="tex">
<input type="button" name="bot" value="Haz click aqui" onclick="copia()">
</form>
</body>
</html>
Saludos.