I faced a similar situation. My development environment was WINDOWS, and it worked perfectly as described in DOCs , i.e. MY_string_helper.php
In a UNIX production environment, he gave an error:
Unable to load requested file: helpers / my_form_helper.php
Changing the file name in lowercase resolved this error BUT , the helper was not loaded.
After repeated squandering, with the help of a hit and a trail he worked [he is not documented]
In autoload.php, just include the reference to the extended helper before the default helper:
$autoload['helper'] = array('my_string', 'string');
In addition, the prefix must have capital; it does not work with the lower case prefix:
$config['subclass_prefix'] = 'MY_';
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