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a bug. Please double-check the documentation available at
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http://php.net/manual/en/language.types.array.php
> Strings containing valid integers will be cast to the integer type. E.g. the key
> "8" will actually be stored under 8.

Having [1] and ["1"] be distinct would create chaos.

"Strings containing valid integers will be cast to the integer type" - yeah that was written BEFORE strict-types existed but don't make any sense in a file which starts with <?php declare(strict_types=1); and calls functions as in my example - that error happend in the real world while step-by-step change a whole application to strict-typing

any by all respect it's idiotic that you cast something which is a string by intention to a integer and force me to do another explicit cast back to string

foreach($password_references as $version=>$hash)
{
 $generated = $cl_api->user_db->generate_password_hash('autotest_pwd', (string)$version);

That's how arrays always worked, one just never really noticed due to us not having a strict mode … [I'm not going to discuss here whether the strict mode was a bad idea …]
Just don't use strict mode then.

All numeric array keys are automatically casted to integers, unfortunately.