Hola Mikil
Código PHP:
<html>
<head>
<script>
function suma() {
textfield3.value = parseFloat(textfield1.value) + parseFloat(textfield2.value)
}
</script>
</head>
<body>
<input type="text" value=0 name="textfield1" onblur=suma()>
<input type="text" value=0 name="textfield2" onblur=suma()>
<input type="text" name="textfield3" readonly="true">
</body>
</html>
Suerte
