Could you tell me how to install the php extension in alpine:3.7 in docker?

how does alpine:3.7 install the php extension in docker?
Environment needs php5.6+nginx
found that there is no mbstring extension below php7, and there is no such extension in alpine repository. How can I install it in docker? Look forward to God"s advice! Thank you

Mar.07,2021
MySQL Query : SELECT * FROM `codeshelper`.`v9_news` WHERE status=99 AND catid='6' ORDER BY rand() LIMIT 5
MySQL Error : Disk full (/tmp/#sql-temptable-64f5-1ea10a6-1b22.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1ea10a6-1b22.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
Need Help?