当用户按下后退按钮时，我想取消设置会话

I have a form which have 9 Steps I have design in this steps using Sessions and Code Whole Form in Php Using Concatenate Method. But Now I am facing a problem i want to allow the user to go on previous step when he click the step back button.

When i am trying to code in on click event php return me unexpected if or unset error. I tried two methods which is written below

 // METHOD 1
 $form .= "<button type='button' class='btn btn-warning' onclick='".unset($_SESSION['m_id']); ."' id='back'>Back</button>";

// METHOD 2 
$form .= "<button type='button' class='btn btn-warning' onclick='".
 if(isset($_SESSION['m_type'])){
 unset($_SESSION['m_type']);
 }
."' id='back'>Back</button>";

I am creating this form in php that is why in the last i echo my form variable.

It is not possible in PHP. You are writing php code like javascript. You have to use AJAX for it. Create one page where you can unset session when click on your button. The way you are trying to do it impossible.