Eso lo puedes hacer con javascript.
Ejemplo:
Código PHP:
<script language="javascript">
function HabilitaBoton(){
if(document.form.email.value.length < 2){
document.form.email.focus();
return 0;
}
if(document.form.pass.value.length < 2){
document.form.pass.focus();
return 0;
}
document.form.guardar.disabled = false;
}
</script>
</head>
<body>
<form action="" method="post" name="form" id="form">
Email: <input name="email" type="text"><br>
Pass: <input name="pass" type="text" onblur="javascript:HabilitaBoton()"><br>
<input name="guardar" value="Guardar" type="button" onClick="javascript:verifica()" disabled><br>
</form>
</body>
Suerte
