Foreign key check setting is not respected

Describe the bug

The setting for foreign key checks has no effect. I checked the phpMyAdmin configuration and it's set to Server default which is:

SHOW VARIABLES;
// ...
foreign_key_checks | ON

I changed the phpMyAdmin setting for the current session to Enable but it's still the same behaviour.

When executing a DROP INDEX query via STRUCTURE tab -> Indexes -> Drop the following form data are sent:

server: 1
table: test_foreign_key
db: test
goto: tbl_structure.php
sql_query: ALTER TABLE `test_foreign_key` DROP INDEX `FOREIGN_KEY`;
message_to_show: Index FOREIGN_KEY has been dropped.
is_js_confirmed: 1
ajax_request: true
fk_checks: 0
_nocache: 1582562296948961610
token: 31607d473d5c724c4b6d3e2d75417562

Furthermore I tried executing the query via the SQL tab. The Enable foreign key checks checkbox is checked. The form data look different, especially the duplicate fk_checks key looks suspicious:

is_js_confirmed: 1
db: test
table: test_foreign_key
token: 31607d473d5c724c4b6d3e2d75417562
pos: 0
goto: tbl_sql.php
message_to_show: Your SQL query has been executed successfully.
prev_sql_query: 
sql_query: ALTER TABLE `test_foreign_key` DROP INDEX `FOREIGN_KEY`
sql_delimiter: ;
fk_checks: 0
fk_checks: 1
SQL: Go
ajax_request: true
ajax_page_request: true
_nocache: 1582562484931112474
token: 31607d473d5c724c4b6d3e2d75417562

To Reproduce

Two test tables:

CREATE TABLE `test_entity` (
 `ID` mediumint(8) unsigned NOT NULL AUTO_INCREMENT,
 PRIMARY KEY (`ID`)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci

CREATE TABLE `test_foreign_key` (
 `ID` int(11) NOT NULL AUTO_INCREMENT,
 `FOREIGN_KEY` mediumint(8) unsigned NOT NULL,
 PRIMARY KEY (`ID`),
 KEY `FOREIGN_KEY` (`FOREIGN_KEY`),
 CONSTRAINT `test_foreign_key_ibfk_1` FOREIGN KEY (`FOREIGN_KEY`) REFERENCES `test_entity` (`ID`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci

Steps to reproduce the behavior:

1. Go to Table Structure
2. Click on "Drop" of "FOREIGN_KEY" index
3. A modal opens, confirm it

Expected behavior

It's not allowed to drop the index because it's used in a foreign key constraint.

Screenshots

Server configuration

• Operating system: Debian
• Web server: Apache 2.4
• Database version: 5.6
• PHP version: 7.4.1
• phpMyAdmin version: 5.0.1

Client configuration

• Browser: Chrome (80.0.3987.116)
• Operating system: Ubuntu 18.04

Reproduce the issue from the Structure tab by dropping the FOREIGN_KEY index on the supplied test tables, then compare the submitted fk_checks values with the SQL tab behavior. Trace the Structure > Indexes > Drop and SQL tab entry points to determine why foreign-key checks are not respected. Done means the drop is rejected when the index is required by the foreign-key constraint.