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run code in 200+ php & hhvm versions
<?php var_dump(setlocale(LC_ALL, 'en_US.UTF-8')); var_dump(str_ireplace("б", "1", "111ббб"));
Output for 5.6.0 - 5.6.30, hhvm-3.18.4 - 3.21.0, 7.0.0 - 7.2.6
bool(false) string(6) "111111"