Php output value

For php output values, I made a simple html login form. click submit to submit for verification asynchronously. The PHP file is connected to the database to verify the user name and password. If yes, an 'true' string is echo and no output value is returned. After returning to html, make a judgment and then respond to the action. However, during the test, I found that no matter whether the error is lost or not, the page is the right action. The php part is correct. you can output the response value based on the data submitted by the form. I think the bug is not correct on the client side. please help me. The following is a client script: