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run code in 150+ php & hhvm versions
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<?php mb_internal_encoding('UTF-8'); echo 'Subject: ', mb_encode_mimeheader('“”‘’ ☕ ♥ ☺ Testing special characters, yo', 'UTF-8', 'B');
based on r9sO2
Output for 4.3.0 - 7.1.0
Subject: =?UTF-8?B?4oCc4oCd4oCY4oCZIOKYlSDimaUg4pi6IFRlc3Rpbmcgc3BlY2lhbCBjaGFy?= =?UTF-8?B?YWN0ZXJzLCB5bw==?=