PHP safe json encode

safe_json_encode.php

<?php
	function safe_json_encode($data) {

		// We might have been tolerant to some common cases such as convert 
		// INF/NAN as 0 by using JSON_PARTIAL_OUTPUT_ON_ERROR option, but
		// sadly `json_last_error()` only get the last error means it may
		// override worse errors such as malfored utf-8 which we can't ignore!
		// Poor H P !!

		$result = @json_encode($data, JSON_UNESCAPED_SLASHES | JSON_UNESCAPED_UNICODE);

		$err = json_last_error();

		// strictly no error
		if ($err === JSON_ERROR_NONE && $result) {
			// Escape </script> to prevent XSS
			// Note: Commonly `str_replace()` is not safe for corrupted string
			// But in our case, `json_encode()` already ensure `$result` as a
			// valid utf-8 string.
			return str_replace('</script>', '<\/script>', $result);
		}

		error_log(
			'json encode error: ' . json_last_error_msg() .
			', trace: ' . print_r(debug_backtrace(), true)
		);

		// When error, PHP 5.5 may return `false`, which output nothing.
		// PHP 5.4 (or PHP 5.5 with JSON_PARTIAL_OUTPUT_ON_ERROR option on)
		// may return "{null: ...}" if the key is a malfored utf-8 string, 
		// which is not valid JSON string.
		// Instead of such meaningless and harmful result which may cause
		// JavaScript error or potential XSS, we return 'null' to denote the
		// failure.
		return 'null';
	}

what is this method doing as a final result? its returning null in the end.

is it testing that $data is valid to convert in json or not