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In PHP, trying to access an empty constant that has not been defined results in an empty string. When a constant is referenced within a script and it doesn't exist, PHP will not throw an error; instead, it treats it as an undefined value, which is effectively equivalent to an empty string.
This behavior is significant because it allows for flexible coding practices, where constants are often used in conditional statements or as configuration values. If a developer attempts to use an empty constant, PHP will return an empty string, which can be useful in situations where the intention is to check for the presence of a value without causing disruption to the flow of the code.
Thus, the output when referencing an empty constant is an empty string, reinforcing the idea that PHP handles undefined constants gracefully by returning a value that allows the script to continue running without interruption.