If encoding of PHP script differs from encoding of string to be processed by mb_ereg_replace(), then you can't just write pattern in script. Both $pattern and $replacement must be converted to same encoding as string to be processed. In this example script is in UTF-8, file to be processed is in UTF-16LE encoding:

<?php
$file_encoding = 'UTF-16LE';
mb_regex_encoding( $file_encoding );

$pattern     = "aaa";
$replacement = "AAA";
$pattern_encoded     = mb_convert_encoding( $pattern,     $file_encoding, 'UTF-8' );
$replacement_encoded = mb_convert_encoding( $replacement, $file_encoding, 'UTF-8' );

$result = mb_ereg_replace( $pattern_encoded, $replacement_encoded, file_get_contents('UTF-16LE.txt') );
file_put_contents('UTF-16LE-updated.txt', $result);
?>