What does the PHP error message "Notice: Use of undefined constant" mean?

What does the PHP error message "Notice: Use of undefined constant" mean?

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PHP Notice: Undefined offset: 1 with array when reading data

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php $_GET and undefined index

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undefined offset PHP error

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Check if variable exist in laravel's blade directive

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PHPStorm: undefined variables caused by include/require

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Undefined offset while accessing array element which exists

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Sending email with laravel, but doesn't recognize variable [duplicate]

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Notice: Undefined index: HTTP_REFERER

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Undefined index error PHP

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How to handle Undefined Offset in laravel?

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Undefined function sha256()

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Undefined variable error in PHP include file [duplicate]

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Is it possible in PHP to prevent "Fatal error: Call to undefined function"?

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PHP and undefined variables strategy

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