Undefined constant error in php 7.2

This is a common warning that occurs whenever PHP has detected the usage of an undefined constant.

Here is an example of constant being defined in PHP:

define('PI', 3.14);

Below is a list of some cases that might cause the issue:

• Forgetting to use a $ symbol at the start of a variable name.

  $name = "Aniket";
  echo name; // forgot to add $ before name
  

  The above code will throw: Notice: Use of undefined constant name – assumed ‘name’. Because there is no dollar sign in front of the variable “name”, PHP assumes that I was trying to reference a constant variable called “name”.

• Forgetting to place quotes around strings.

  echo $_POST[email];
  

  In the example above, I failed to place quotes around the $_POST variable “email”. This code will throw: Notice: Use of undefined constant name – assumed ’email’.

  To fix this, I’d obviously have to do the following:

  echo $_POST["email"];
  

According to Deprecated features in PHP 7.2.x you should not use undefined constants because:

Unquoted strings that are non-existent global constants are taken to be strings of themselves.

This behaviour used to emit an E_NOTICE, but will now emit an E_WARNING