<form method="post" action="">
    <input type="text" name="a">
    <input type="text" name="b">
    <input type="text" name="c">
    <input type="text" name="d">
    <input type="submit">
</form>


<script>
    var Bdom = document.createElement('span');
    var input = document.getElementsByTagName('input');
    var _input = [];
    for (var k in input )
    {
        if(input[k].name)_input[input[k].name] = input[k];
        if(input[k].name == 'c') 
            input[k].parentNode.insertBefore(Bdom, input[k]);
    }

    _input['a'].onkeyup = function ()
    {
            Bdom.innerHTML = (this.value == 'PHP') ? '*php' : '';
    }

    _input['b'].onkeyup = function ()
    {
        if(this.value == '3')
            _input['c'].value = 3*0.5;
    }
 
</script>