无法找到解决方案,我该如何解决

Fatal error: Cannot use isset() on the result of an expression (you can use "null !== expression" instead) in E:\xampp\htdocs\loginregister\profile.php on line 15

I use it to get msg from my database file

<?php if (isset($_GET('msg') )) { echo "<span style='color:green'>".$_GET('msg')."</span>"; } ?>

$_GET is an associative array of variables passed to the current script via the URL parameters.

So you have to access the msg like $_GET['msg']

Try this:

<?php 

if ( isset( $_GET['msg'] ) ) { 
      echo "<span style='color:green'>" . $_GET['msg'] . "</span>"; 
} 

?>

Doc: http://php.net/manual/en/reserved.variables.get.php

we can not use an expression in isset.

The isset () function is used to check whether a variable is set or not.

for example if(isset($_REQUEST['submit']))

if you want to use expression the you should use if statmet like

if($_REQUEST['submit']==''){}else{}

or

if($result >=0 or $result ==''){}else{}

if you want a specific answer, please mention your condition