I have a question concerning UTF-8 and htmlentities. I have two variables with greek text, both of them seem to be UTF-8 encoded (according to mb_detect_encoding()). When I output the two variables, they look exactly the same in the browser (also in the source code).
I was astonished when I realized, that a simple if($var1 == $var2) always failed although they seemed to be exactly the same. So I used htmlentities to see whether the html code would be the same. I was surprised when I saw that the first variable looked like this: Ï�κÏ�λοÏ� and the other one like this: ια&ro;. How can it be that two identical words with the same encoding (UTF-8) are nevertheless different? And how could I fix this problem?
Your first question was: How can it be that two identical words with the same encoding (UTF-8) are nevertheless different?
In this case, the encoding isn't really UTF-8 in both cases. The first variable is in "real" UTF-8, while in the second, greek characters are not really in UTF-8, but in ASCII, with non-ASCII characters (greek) encoded using something called a CER (Character Entity Reference).
A web browser and some too friendly "WYSIWYG" editors will render these strings as identical, but the binary representations of the actual strings (which is what the computer will compare) are different. This is why the equal test fails, even if the strings appear to be the same upon human visual ispection in a browser or editor.
I don't think you can rely on mb_detect_encoding to detect encoding in such cases, since there is no way of telling utf-8 apart from ASCII using CER to represent non-ASCII.
Your second question was: How could I fix this problem?
Before you can compare strings that may be encoded differently, you need to convert them to canonical form ( Wikipedia: Canonicalization ) so that their binary representation is identical.
Here is how I've solved it: I've implemented a handy function named utf8_normalize that converts just about any common character representation (in my case: CER, NER, iso-8859-1 and CP-1252) to canonical utf-8 before comparing strings. What you throw in there must to some extent be determined by what are "popular" character representations in the type of environment your software will operate, but if you just make sure that your strings are on canonical form before comparing, it will work.
As noted in the comment below from the OP (phpheini), there also exists the PHP Normalizer class, which may do a better job of normalization that a home-grown function.
Related
The PHP documentation says:
Of course, in order to be useful, functions that operate on text may have to make some assumptions about how the string is encoded. Unfortunately, there is much variation on this matter throughout PHP’s functions:
[... a few special cases are described ...]
Ultimately, this means writing correct programs using Unicode depends on carefully avoiding functions that will not work and that most likely will corrupt the data [...]
Source: https://www.php.net/manual/en/language.types.string.php
So naturally my question is: Where are these specifications that allow us to identify the encoding/charset associated to string arguments, return values, constants, array keys/values, ... for built-in functions/methods/data (e.g. array_key_exists, DOMDocument::getElementsByTagName, DateTime::format, $_GET[$key], ini_set, PDO::__construct, json_decode, Exception::getMessage() and many more)? How do composer package providers specify the encodings in which they accept/provide textual data?
I have been working roughly with the following heuristic: (1) never change the encoding of anything, (2) when forced to pick an encoding, pick UTF-8. This has been working for years but it feels very unsatisfactory.
Whenever I try to find an answer to the question, I only get search results relating to url encoding, HTML entities or explaining the interpretation of string literals (with the source file's encoding).
Strings in PHP are what other languages would call byte arrays, i.e. purely a raw sequence of bytes. PHP is not generally interested in what characters those bytes represent, they're just bytes. Only functions that need to work with strings on a character level need to be aware of the encoding, anything else doesn't.
For example, array_key_exists doesn't need to know anything about characters to figure out whether a key with the same bytes as the given string exists in an array.
However, mb_strlen for example explicitly tells you how many characters the string consists of, so it needs to interpret the given string in a specific encoding to give you the right number of characters. mb_strlen('漢字', 'latin1') and mb_strlen('漢字', 'utf-8') give very different results. There isn't a unified way how these kinds of functions are made encoding aware*, you will need to consult their manual entries.
* The mb_ functions in particular generally use mb_internal_encoding(), but other sets of functions won't.
Functions like DateTime::format are looking for specific characters in the format string to replace by date values, e.g. d for the day, m for the month etc. You can generally assume that these are ASCII byte values it's looking for, unless specified otherwise (and I'm not aware of anything that specifies otherwise). So any ASCII compatible encoding will usually do.
For a lot more details, you may be interested in What Every Programmer Absolutely, Positively Needs To Know About Encodings And Character Sets To Work With Text.
Often this can be found in the official documentation, e.g., the DOMDocument class has a property encoding (determined by XML declaration). As for methods that return strings, I recommend reading this
I'm looking increasingly into ensuring that PHP apps are multibyte-safe, which mostly involves replacing string manipulation functions with their equivilant mb_* functions.
However string concatenation is giving me pause for thought.
Some character encodings (such as UTF-16 unicode) can include a Byte Order Mark at the beginning. If you concatenated two UTF16 strings it's possible you'd introduce a BOM into the resulting string at a location other than the beginning. I suspect that there are other encodings that can also include "header" information such that stitching two strings of the same encoding together would also be problematic. Is PHP smart enough to discard BOMs etc when doing multibyte string concatenations? I suspect not because PHP has traditionally only treated strings as a sequence of bytes. Is there a multibyte-safe equivalent to concatenation? I've not been able to find anything in the mbstring documentation.
Obviously it would never be safe to concatenate strings that are in different encodings so I'm not worrying about that for now.
PHP has traditionally only treated strings as a sequence of bytes
It still does. PHP has no concept of a character string, as it exists in other languages. Therefore, all strings are always byte strings and you will need to manually track which of them are binary strings, which are character strings and which encoding is being used. An effort to bring Unicode strings to PHP resulted in PHP 6, which was abandoned and never released. But then again, even languages which do have native character strings will not automatically do what you're asking for anyway.
Have a look at the Unicode FAQ about BOM, some of the information below directly comes from there.
If a byte order mark ends up in the middle of a string, Unicode dictates that it should be interpreted as ZERO WIDTH NON-BREAKING SPACE. I conclude that this shouldn't usually be an issue, so it's not so terrible to just ignore BOMs.
However, if that bothers you, my recommendation is as follows:
Try to avoid BOMs altogether and mark the data stream accordingly. For example, when using HTTP, set the encoding to UTF-16BE or UTF-16LE using headers.
Sanitize all inputs used by the application (user input, files loaded, ...) as early as possible, by removing these BOMs and converting the encoding. You may even want to use the Normalizer class. Use your favorite framework's features if available.
Use one and only one encoding internally. Use mb_internal_encoding() to set a default for all mb_*() functions.
When outputting strings, add any desired BOMs back to the strings if you must. Again, it's preferable to just mark the data stream correctly.
That said, note that concatenating multi-byte strings can cause a multitude of unexpected situations, the BOM in the middle of a string is just one of them. Issues may also arise when using bidirectional text where RTL or LTR code points in the first string being concatenated may affect text in the second string. Further, many issues may also arise when using other string operations, for example using mb_substr() on bidirectional text may also produce unexpected results. Text involving combining diacritical marks may also be problematic.
I've looked across the web, I've looked through SO, through PHP documentation and more.
It seems like a ridiculous problem not to have a standard solution to. If you get an unknown character set, and it has strange characters (like english quotes), is there a standard way to convert them to UTF-8?
I've seen many messy solutions using a plethora of functions and checking and none of them are definitely going to work.
Has anyone come up with their own function or a solution that always works?
EDIT
Many people have answered saying "it is not solvable" or something of that nature. I understand that now, but none have given any sort of solution that has worked besides utf8_encode which is very limited. What methods ARE out there to deal with this? What is the best method?
No. One should always know what character set a string is in. Guessing the character set by using a sniffing function is unreliable (although in most situations, in the western world, it's usually a mix-up between ISO-8859-1 and UTF-8).
But why do you have to deal with unknown character sets? There is no general solution for this because the general problem shouldn't exist in the first place. Every web page and data source can and should have a character set definition, and if one doesn't, one should request the administrator of that resource to add one.
(Not to sound like a smartass, but that is the only way to deal with this well.)
The reason why you saw so many complicated solutions for this problem is because by definition it is not solvable. The process of encoding a string of text is non-deterministic.
It is possible to construct different combinations of text and encodings that result in the same byte stream. Therefore, it is not possible, strictly logically speaking, to determine the encoding, character set, and the text from a byte stream.
In reality, it is possible to achieve results that are "close enough" using heuristic methods, because there is a finite set of encodings that you'll encounter in the wild, and with a large enough sample a program can determine the most likely encoding. Whether the results are good enough depends on the application.
I do want to comment on the question of user-generated data. All data posted from a web page has a known encoding (the POST comes with an encoding that the developer has defined for the page). If a user pastes text into a form field, the browser will interpret the text based on encoding of the source data (as known by the operating system) and the page encoding, and transcode it if necessary. It is too late to detect the encoding on the server - because the browser may have modified the byte stream based on the assumed encoding.
For instance, if I type the letter Ä on my German keyboard and post it on a UTF-8 encoded page, there will be 2 bytes (xC3 x84) that are sent to the server. This is a valid EBCDIC string that represents the letter C and d. This is also a valid ANSI string that represents the 2 characters à and „. It is, however, not possible, no matter what I try, to paste an ANSI-encoded string into a browser form and expect it to be interpreted as UTF-8 - because the operating system knows that I am pasting ANSI (I copied the text from Textpad where I created an ANSI-encoded text file) and will transcode it to UTF-8, resulting in the byte stream xC3 x83 xE2 x80 x9E.
My point is that if a user manages to post garbage, it is arguably because it was already garbage at the time it was pasted into a browser form, because the client did not have the proper support for the character set, the encoding, whatever.
Because character encoding is non-deterministic, you cannot expect that there exist a trivial method to uncover from such a situation.
Unfortunately, for uploaded files the problem remains. The only reliable solution that I see is to show the user a section of the file and ask if it was interpreted correctly, and cycle through a bunch of different encodings until this is the case.
Or we could develop a heuristic method that looks at the occurance of certain characters in various languages. Say I uploaded my text file that contains the two bytes xC3 x84. There is no other information - just two bytes in the file. This method could find out that the letter Ä is fairly common in German text, but the letters à and „ together are uncommon in any language, and thus determine that the encoding of my file is indeed UTF-8. This roughy is the level of complexity that such a heuristic method has to deal with, and the more statistical and linguistic facts it can use, the more reliable will its results be.
Pekka is right about the unreliability, but if you need a solution and are willing to take the risk, and you have the mbstring library available, this snippet should work:
function forceToUtf8($string) {
if (!mb_check_encoding($string)) {
return false;
}
return mb_convert_encoding($string, 'UTF-8', mb_detect_encoding($string));
}
If I'm not wrong, there is something called utf8encode... it works well EXCEPT if you are already in utf8
http://php.net/manual/en/function.utf8-encode.php
I would like to detect encoding of some text (using PHP).
For that purpose i use mb_detect_encoding() function.
The problem is that the function returns different results if i change the order of possible encodings with mb_detect_order() function.
Consider the following example
$html = <<< STR
ちょっとのアクセスで落ちてしまったり、サーバー障害が多いレンタルサーバーを選ぶとあなたのビジネス等にかなりの影響がでてしまう可能性があります。特に商売をされている個人の方、法人の方は気をつけるようにしてください
STR;
mb_detect_order(array('UTF-8','EUC-JP', 'SJIS', 'eucJP-win', 'SJIS-win', 'JIS', 'ISO-2022-JP','ISO-8859-1','ISO-8859-2'));
$originalEncoding = mb_detect_encoding($str);
die($originalEncoding); // $originalEncoding = 'UTF-8'
However if you change the order of encodings in mb_detect_order() the results will be different:
mb_detect_order(array('EUC-JP','UTF-8', 'SJIS', 'eucJP-win', 'SJIS-win', 'JIS', 'ISO-2022-JP','ISO-8859-1','ISO-8859-2'));
die($originalEncoding); // $originalEncoding = 'EUC-JP'
So my questions are:
Why is that happening ?
Is there a way in PHP to correctly and unambiguously detect encoding of text ?
That's what I would expect to happen.
The detection algorithm probably just keeps trying, in order, the encodings you specified in mb_detect_order and then returns the first one under which the bytestream would be valid.
Something more intelligent requires statistical methods (I think machine learning is commonly used).
EDIT: See e.g. this article for more intelligent methods.
Due to its importance, automatic charset detection is already implemented in major Internet applications such as Mozilla or Internet Explorer. They are very accurate and fast, but the implementation applies many domain specific knowledges in case-by-case basis. As opposed to their methods, we aimed at a simple algorithm which can be uniformly applied to every charset, and the algorithm is based on well-established, standard machine learning techniques. We also studied the relationship between language and charset detection, and compared byte-based algorithms and character-based algorithms. We used Naive Bayes (NB) and Support Vector Machine (SVM).
Not really. The different encodings often have large areas of overlap, and if your string that you are testing exists entirly inside that overlap, then both encoding are acceptable.
For example, utf-8 and ISO-8859-1 are the same for the letters a-z. The string "hello" would have an identical sequence of bytes in both encodings.
This is exactly why there is an mb_detect_order() function in the first place, as it allows you to say what you would prefer to happen when these clashes happen. Would you like "hello" to be utf-8 or ISO-8859-1?
Keep in mind mb_detect_encoding() does not know what encoding the data is in. You may see a string, but the function itself only sees a stream of bytes. Going by that, it needs to guess what the encoding is - e.g. ASCII would be if bytes are only in the 0-127 range, UTF-8 would be if there are ASCII bytes and 128+ bytes that exist only in pairs or more, and so forth.
As you can imagine, given that context, it's quite difficult to detect an encoding reliably.
Like rihk said, this is what the mb_detect_order() function is for - you're basically supplying your best guess what the data is likely to be. Do you work with UTF-8 files frequently? Then chances are your stuff isn't likely to be UTF-16 even if mb_detect_encoding() could guess it as that.
You might also want to check out Artefacto's link for a more in-depth view.
Example case: Internet Explorer uses some interesting encoding guessing if nothing is specified (#link, Section: 'To automatically detect a website's language') that's caused strange behaviours on websites that took encoding for granted in the past. You can probably find some amusing stuff on that if you google around. It makes for a nice show-case how even statistical methods can backfire horribly, and why encoding-guessing in general is problematic.
mb_detect_encoding looks at the first charset entry in your mb_detect_order() and then loops through your input $html matching character by character whether that character falls within the valid set of characters for the charset. If every character matches, then it returns true; if any character fails, it moves on to the next charset in the mb_detect_order() and tries again.
The wikipedia list of charsets is a good place to see the characters that make up each charset.
Because these charset values overlap (char x8fA1EF exists in both 'UTF-8' and in 'EUC-JP') this will be considered a match even though it's a totally different character in each character set. So unless any of the character values exist in one charset, but not in another, then mb_detect_encoding can't identify which of the charsets is invalid; and will return the first charset from your array list which could be valid.
As far as I'm aware, there is no surefire way of identifying a charset. PHP's "best guess" method can be helped if you have a reasonable idea of what charsets you are likely to encounter, and order your list accordingly based on the gaps (invalid characters) in each charset.
The best solution is to "know" the charset. If you are scraping your html from another page, look for the charset identifier in the header of that page.
If you really want to be clever, you can try and identify the language in which the html is written, perhaps using trigrams or n-grams or similar as described in this article on PHP/ir.
About 2 years ago I made the mistake of starting a large website using iso-8859-1. I now am having issues with some characters, especially when sending data to the server using ajax. Because of this, I would like to switch to using UTF-8.
What issues do you see coming from this? I know I would have to search the site to look for characters that need to be changed from a ? to their real characters. But, are there any other risks in doing this? Has anyone done this before?
The main difficulty is making sure you've checked that all the data paths are UTF-8 clean:
Is your site DB-backed? If so, you'll need to convert all the tables to UTF-8 or some other Unicode encoding, so sorting and text searching work correctly.
Is your site using some programming language for dynamic content? (PHP, mod_perl, ASP...?) If so, you'll have to make sure the particular language interpreter you're using fully understands some form of Unicode, work out the conversions if it isn't using UTF-8 natively — UTF-16 is next most common — and check that it's configured to use UTF-8 on its output to the web server.
Does your site have some kind of back-end app server? Does it use UTF-8 for its text outputs?
There are at least three different places you can declare the charset for a web document. Be sure you change them all:
the HTTP Content-Type header
the <meta http-equiv="Content-Type"> tag in your documents' <head>
the <?xml> tag at the top of the document, if using XHTML Strict
All this comes from my experiences a years ago when I traced some Unicode data through a moderately complex N-tier app, and found conversion chains like:
Latin-1 → UTF-8 → Latin-1 → UTF-8
So, even though the data ended up in the browser claiming to be "UTF-8", the app could still only handle the subset common with Latin-1.
The biggest reason for those odd conversion chains was due to immature Unicode support in the tooling at the time, but you can still find yourself messing with ugliness like this if you're not careful to make the pipeline UTF-8 clean.
As for your comments about searching out Latin-1 characters and converting files one by one, I wouldn't do that. I'd build a script around the iconv utility found on every modern Linux system, feeding in every text file in your system, explicitly converting it from Latin-1 to UTF-8. Leave no stone unturned.
Such a change touches (nearly) every part of your system. You need to go through everything, from the database to the PHP to the HTML to the web browser.
Start a test site and subject it to some serious testing (various browsers on various platforms doing various things).
IMO it's important to actually get familiar with UTF-8 and what it means for software. A few quick points:
PHP is mostly byte-oriented. Learn the difference between characters and code points and bytes, and between UTF-8 and Unicode.
UTF-8 is well-designed. For instance, given two UTF-8 strings, a byte-oriented strstr() will still function correctly.
The most common problem is treating a UTF-8 string as ISO-8859-1 and vice versa - you may need to add documentation to your functions stating what kind of encoding they expect, to make these sorts of errors less likely. A variable naming convention for your strings (to indicate what encoding they use) may also help.