Código PHP:
<html>
<head>
<title>Untitled</title>
<script>
function comprobar(){
if(valor.value){
decimal=(valor.value-parseInt(valor.value));
decimal=parseInt(Math.ceil(1000*decimal/6))/100;
todo=parseInt(valor.value)+decimal;
a=Math.floor(todo*4)/4;
alert(a)}
}
</script>
</head>
<body>
<input type="text"
name="valor"
size="5"
maxlength="5" onBlur='if(isNaN(this.value)||this.value>99){this.value="";this.focus()}'>
<input type="button"
value="pincha"
onclick="comprobar()"
>
</body>
</html>