Time-based mysql injection penetration testing tool - sleep() not working

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MySQL

Hello,

I'm creating a penetration testing tool to scan for a variety of sql injection vulnerabilities. A simple starting example I'm failing to get working is the following;

http://xxx/update_comment.php?id=test';select case when (select substring(table_name, 1, 1) from information_schema.tables limit 1) = 'b' then 1 else sleep(3) end#

This is the relevant line of my test script which is causing problems;

$conn->query('UPDATE test_table SET id = id WHERE id = \'' . $_GET['id'] . '\'');

When I execute the command which would be executed by the script, in Adminer, it works. But when the script tries it, it returns immediately and as such, no timings can be gathered from the test. What could be causing this immediate return?

What I have tried:

Changing the sleep time
Asking google, to no avail