Description:
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Defining constants with invalid characters (e.g. leading numbers) are allowed, define() returns true, checking the constant with defined() also returns true, 
but when you try to use it you have a parse error.

Test script:
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var_dump ( define ("0000X" , 1) );

var_dump ( defined ("0000X") );

echo 0000X;

http://sandbox.onlinephpfunctions.com/code/a7e955c09fbcc9e17a51ea56a75c70f8f60ab7e7

Expected result:
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var_dump ( define ("0000X" , 1) ); // false, throwing an notice or a warning

var_dump ( defined ("0000X") ); // false

echo 0000X; // Parse error


Actual result:
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var_dump ( define ("0000X" , 1) ); // true

var_dump ( defined ("0000X") ); // true

echo 0000X; // Parse error