Why does codeigniter & Linux server throws unserialize() session data error when user tries to login

PHP Snippet 1:

$recalculated_data = preg_replace_callback( '!s:(\d+):"(.*?)";!', function( $match ) {      
    return ( $match[1] == strlen( $match[2] ) ) ? $match[0] : 's:' . strlen( $match[2] ) . ':"' . $match[2] . '";';
}, $data );

unserialize( $recalculated_data );