What will happen if you try to access an undefined constant in PHP?

When you attempt to access an undefined constant in PHP, the language will generate a warning. PHP tries to interpret the undefined constant as a string by taking its name and converting it to a string. If the constant name does not exist, PHP issues a warning to inform you of this issue, but the script will continue to execute.

For example, if you try to access MY_CONSTANT and it has not been defined using the define() function or as a constant in a class, PHP interprets it as a string 'MY_CONSTANT'. Consequently, the warning provides a soft alert of the problem while allowing the rest of the code to run.

Understanding this behavior is essential for debugging and maintaining robust code in PHP applications. It highlights the importance of constant definitions and the attention needed for potential typos or overlooked definitions in your code.