Question: In the function [mb\_detect\_encoding](http://php.net/manual/en/function.mb-detect-encoding.php) there is a parameter for strict mode.

In the first, most upvoted comment:

```
<?php
$str = 'áéóú'; // ISO-8859-1
mb_detect_encoding($str, 'UTF-8'); // 'UTF-8'
mb_detect_encoding($str, 'UTF-8', true); // false

```

This is true, yes. But can anybody give me an explanation, why is it?

Answer: 
Everything in this answer is based on my reading of the code [here](https://github.com/php/php-src/blob/c72282a13b12b7e572469eba7a7ce593d900a8a2/ext/mbstring/libmbfl/mbfl/mbfilter.c#L718) and [here](https://github.com/php/php-src/blob/c72282a13b12b7e572469eba7a7ce593d900a8a2/ext/mbstring/libmbfl/filters/mbfilter_utf8.c#L243).

I did not write it, I did not step through it with a debugger, this is my interpretation only.

---

It seems that the *intention* was for strict mode to check if the string as a whole was valid for the encoding, while non-strict mode would allow for a sub-sequence that *could be part of a valid string.* For example, if the string ended with what should be the first byte of a multi-byte character it would not match in strict mode but would still qualify as UTF-8 under non-strict mode.

However there seems to be a bug\* where in non-strict mode only the first byte of the string is being checked in some circumstances.

**Example:**

The byte `0xf8` is not allowed anywhere in UTF-8. When placed at the start of a string `mb_detect_encoding()` properly returns false for it regardless of which mode is used.

```
$str = "\xf8foo";

var_dump(
    mb_detect_encoding($str, 'UTF-8'),      // bool(false)
    mb_detect_encoding($str, 'UTF-8', true) // bool(false)
);

```

But as long as the leading byte may occur anywhere in a UTF-8 sequence, non-strict mode returns UTF-8.

```
$str = "foo\xf8";

var_dump(
    mb_detect_encoding($str, 'UTF-8'),      // string(5) "UTF-8"
    mb_detect_encoding($str, 'UTF-8', true) // bool(false)
);

```

---

So while your ISO-8859-1 string `'áéóú'` is not valid UTF-8, the first byte `"\xe1"` can occur in UTF-8 and `mb_detect_encoding()` mistakenly returns the string as such.

---

\*I've opened a report for this at <https://bugs.php.net/bug.php?id=72933>
`áéóú` in ISO-8859-1 encodes as:

```
e1 e9 f3 fa

```

If you mis-interpret it as UTF-8 you only get four invalid byte sequences. The Multi-Byte extension is basically designed to ignore errors. For instance, `mb_convert_encoding()` will replace those sequences with [question marks](http://www.fileformat.info/info/unicode/char/003f/index.htm) or whatever you set with `mb_substitute_character()`.

My educated guess is that strict encoding determines what should be done with invalid byte sequences:

* `false` means to remove them
* `true` means to keep them

If you ignore these invalid sequences you're obviously discarding extremely valuable information and you only get sensible results in very limited circumstances, e.g.

```
$str = chr(81);
var_dump( mb_detect_encoding($str, ['ISO-8859-1', 'Windows-1252']) );
var_dump( mb_detect_encoding($str, ['Windows-1252', 'ISO-8859-1']) );

```

To sum up, `mb_detect_encoding()` is in general not as useful as you may thing and it's total crap with the default parameters.