How to write the code for the back button?

If you want to do it (what I think you are trying right now) then replace this line

<input type="submit" <a href="#" onclick="history.back();">"Back"</a>

with this

<button type="button" onclick="history.back();">Back</button>

If you don't want to rely on JavaScript then you could get the HTTP_REFERER variable an then provide it in a link like this:

<a href="<?php echo $_SERVER['HTTP_REFERER'] ?>">Back</a>

<a href="javascript:history.back(1)">Back</a>

this one (by Eiko) is perfect, use css to make a button of <a>... eg you can use this css class in <a> as `

<a class=".back_button" href="javascript:history.back(1)">Back</a>`

.back_button {
display:block;
width:100px;
height:30px;
text-align:center;
background-color:yellow;
border:1px solid #000000;
}

You need to tell the browser you are using javascript:

<a href="javascript:history.back(1)">Back</a> 

Also, your input element seems out of place in your code.

<button onclick="history.go(-1);">Back </button>