Only variable references should be returned by reference - Codeigniter

This error message in Codeigniter indicates that you are attempting to return a variable by reference, but only variable references can be returned by reference.

In PHP, you can return a reference to a variable by prefixing the variable name with an ampersand (&) in the function definition. However, you can only return a reference to a variable, not to a literal or an expression.

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Here's an example of correct usage:

<?php

function &get_instance()
{
    return $this;
}

Here's an example of incorrect usage that would trigger the error message:

<?php

function &get_instance()
{
    return 5 + 10; // cannot return a reference to an expression
}

If you are seeing this error message, it means that you are attempting to return a reference to something that is not a variable. To fix the error, you will need to modify your code to ensure that you are only returning references to variables.