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run code in 200+ php & hhvm versions
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<?php var_dump(mb_substr('Hello I am a string', 6, NULL, 'UTF-8')); var_dump(mb_substr('Hello I am a string', 6, NULL)); var_dump(mb_substr('Hello I am a string', 6));
based on UPkbs
Output for 5.4.8 - 7.2.0
string(13) "I am a string" string(13) "I am a string" string(13) "I am a string"
Output for 4.3.0 - 5.4.7
string(0) "" string(0) "" string(13) "I am a string"