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run code in 200+ php & hhvm versions
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<?php define("\\\\a", 20); var_dump(constant("\\\\a"));
based on fdlIO
Output for 5.3.0 - 7.2.0
Warning: constant(): Couldn't find constant \\a in /in/D2uYK on line 6 NULL
Output for 4.3.0 - 5.2.17
int(20)