isset function in php
<!DOCTYPE html> <html lang="en"> <head> <meta charset="UTF-8"> <meta http-equiv="X-UA-Compatible" content="IE=edge"> <meta name="viewport" content="width=device-width, initial-scale=1.0"> <title>Document</title> </head> <body> <form action="" method="post"> <h1>ISSET Demo</h1> <input type="submit" value="Click here" name="btn1"> </form> </body> </html> <?php if(isset($_POST['btn1'])) { echo "button was set!"; } ?>