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<?php class ß {} class s {} class SS {} var_dump( strtoupper("ß"), mb_convert_case("ß", MB_CASE_UPPER, "UTF-8"), mb_strtoupper("ß", "UTF-8") );
Output for 7.3.0 - 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(2) "ß" string(2) "SS" string(2) "SS"
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 - 5.4.45, 5.5.0 - 5.5.38, 5.6.0 - 5.6.40, 7.0.0 - 7.0.33, 7.1.0 - 7.1.33, 7.2.0 - 7.2.33
string(2) "ß" string(2) "ß" string(2) "ß"

preferences:
236.25 ms | 404 KiB | 453 Q