If Isset $_Post

If isset $_POST

Most form inputs are always set, even if not filled up, so you must check for the emptiness too.

Since !empty() is already checks for both, you can use this:

if (!empty($_POST["mail"])) {
    echo "Yes, mail is set";    
} else {  
    echo "No, mail is not set";
}

Php if($_POST) vs if(isset($_POST))

isset determine if a variable is set and is not NULL. $_POST will always be set and will always be an array.

Without isset you are just testing if the value is truthy. An empty array (which $_POST will be if you aren't posting any data) will not be truthy.

php if isset post and else post

You can do something like:

if (isset($_POST['submit']))
{
    // do something here
} 
elseif (isset($_POST['cancel']))
{
    // redirect here
}

if(isset($_POST['submit'])) coding form action is not working

When you want to pass data from form to your php script, you have to remember that the name attribute is assigned for that data to find in php script.

<button type="submit" class="btn btn-info">Submit</button>

In your button you are not assigned the name attribute. So after post method in php script isset($_POST['submit']) does not find any submit attribute. So it returns false.

<button type="submit" class="btn btn-info" name="submit" value="submit" >Submit</button>

So you have to use this line for button.

if (isset($_post['submit'])) is not working

Replace

 <input type="submit" name="Submit" value="Submit" />

with

 <input type="submit" name="submit" value="submit" />