make a simple php form with ajex, without reload the page

i want to make a simple php form there will be one input only, something like that if ($_SERVER["REQUEST_METHOD"] == "POST") { $a will be user here} <form>input variable $a here </form> when input $ a will be enter and we will click on submit button then it will php server code and it there wil be a funtion in that if funtion and input $a will be used and it will show result in front end , when we clicked on submit from form then it will give result though php form in front end it should be done thorugh ajex, page should not be reload and also sometime when we try to reload then it give us warning , we should not see the warning, when we reload then input variable or resulted output should not be there