It looks like your WordPress plugin adds a function with the same name.

As a temporary workaround, instead of renaming init.php, comment/remove this code at the bottom of the file:

/* Custom mb_ucfirst() function - eval'd so we can put into global namespace */
eval( '
function mb_ucfirst()
{
	$text = \func_get_arg( 0 );
	return mb_strtoupper( mb_substr( $text, 0, 1 ) ) . mb_substr( $text, 1 );
}
');

Otherwise, editing the code to this should work, too:

/* Custom mb_ucfirst() function - eval'd so we can put into global namespace */
if ( !\function_exists('mb_ucfirst') )
{
	eval( '
function mb_ucfirst()
{
	$text = \func_get_arg( 0 );
	return mb_strtoupper( mb_substr( $text, 0, 1 ) ) . mb_substr( $text, 1 );
}
');
}

IPS can implement a fix to check if the function exists before running the eval() code. This function is included with PHP 8.4.0+, so it can be removed then. However, since v4 doesn't support PHP 8.2.+, we'll eventually get there with v5.0.0.

Edited January 29Jan 29 by teraßyte

You would need to post this as feedback, as its not technically a bug, and also isnt version 5 (you posted in the v5 bug tracker)