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<?php var_dump(mb_substr('HelloI am a string', 6, NULL, 'UTF-8')); var_dump(mb_substr('HelloI am a string', 6, NULL)); var_dump(mb_substr('HelloI am a string', 6));
Output for 5.4.8 - 5.4.45, 5.5.24 - 5.5.35, 5.6.7 - 5.6.28, 7.0.0 - 7.0.20, 7.1.0 - 7.1.20, 7.2.0 - 7.2.33, 7.3.16 - 7.3.33, 7.4.0 - 7.4.33, 8.0.0 - 8.0.30, 8.1.0 - 8.1.27, 8.2.0 - 8.2.17, 8.3.0 - 8.3.4
string(12) " am a string" string(12) " am a string" string(12) " am a string"
Output for 4.3.0 - 4.3.11, 4.4.0 - 4.4.9, 5.0.0 - 5.0.5, 5.1.0 - 5.1.6, 5.2.0 - 5.2.17, 5.3.0 - 5.3.29, 5.4.0 - 5.4.7
string(0) "" string(0) "" string(12) " am a string"

preferences:
219.52 ms | 403 KiB | 309 Q