That’s a very common situation in PHP world: you try to apply strtolower() or strtoupper() functions to a UTF-8 encoded string, and… You realize you’re fucked up. Special characters are fucked, each one magically turned into fucking �’s, and before you start to fucking panic, fast as a fucking bobcat, you fucking Google your problem. Those 0.4 seconds of wait before the search results show up, suddenly becomes an eternity.

The problem

While there are many languages in which every necessary character can be represented by a one-to-one mapping to an 8-bit value, there are also several languages which require so many characters for written communication that they cannot be contained within the range a mere byte can code (A byte is made up of eight bits. Each bit can contain only two distinct values, one or zero. Because of this, a byte can only represent 256 unique values (two to the power of eight)). Multibyte character encoding schemes were developed to express more than 256 characters in the regular bytewise coding system.

When you manipulate (trim, split, splice, etc.) strings encoded in a multibyte encoding, you need to use special functions since two or more consecutive bytes may represent a single character in such encoding schemes. Otherwise, if you apply a non-multibyte-aware string function to the string, it probably fails to detect the beginning or ending of the multibyte character and ends up with a corrupted garbage string that most likely loses its original meaning.

The solution: Multibyte String Functions

mbstring provides multibyte specific string functions that help you deal with multibyte encodings in PHP. In addition to that, mbstring handles character encoding conversion between the possible encoding pairs. mbstring is designed to handle Unicode-based encodings such as UTF-8 and UCS-2 and many single-byte encodings for convenience.

Conclusion

When dealing with Unicode-base encodings (like UTF-8), always use multibyte string functions. In general, all you have to do is add a mb_ before your function call. Here you can see a list of PHP multibyte functions:

• mb_check_encoding — Check if the string is valid for the specified encoding
• mb_convert_case — Perform case folding on a string
• mb_convert_encoding — Convert character encoding
• mb_convert_kana — Convert “kana” one from another (“zen-kaku”, “han-kaku” and more)
• mb_convert_variables — Convert character code in variable(s)
• mb_decode_mimeheader — Decode string in MIME header field
• mb_decode_numericentity — Decode HTML numeric string reference to character
• mb_detect_encoding — Detect character encoding
• mb_detect_order — Set/Get character encoding detection order
• mb_encode_mimeheader — Encode string for MIME header
• mb_encode_numericentity — Encode character to HTML numeric string reference
• mb_encoding_aliases — Get aliases of a known encoding type
• mb_ereg_match — Regular expression match for multibyte string
• mb_ereg_replace_callback — Perform a regular expresssion seach and replace with multibyte support using a callback
• mb_ereg_replace — Replace regular expression with multibyte support
• mb_ereg_search_getpos — Returns start point for next regular expression match
• mb_ereg_search_getregs — Retrieve the result from the last multibyte regular expression match
• mb_ereg_search_init — Setup string and regular expression for a multibyte regular expression match
• mb_ereg_search_pos — Returns position and length of a matched part of the multibyte regular expression for a predefined multibyte string
• mb_ereg_search_regs — Returns the matched part of a multibyte regular expression
• mb_ereg_search_setpos — Set start point of next regular expression match
• mb_ereg_search — Multibyte regular expression match for predefined multibyte string
• mb_ereg — Regular expression match with multibyte support
• mb_eregi_replace — Replace regular expression with multibyte support ignoring case
• mb_eregi — Regular expression match ignoring case with multibyte support
• mb_get_info — Get internal settings of mbstring
• mb_http_input — Detect HTTP input character encoding
• mb_http_output — Set/Get HTTP output character encoding
• mb_internal_encoding — Set/Get internal character encoding
• mb_language — Set/Get current language
• mb_list_encodings — Returns an array of all supported encodings
• mb_output_handler — Callback function converts character encoding in output buffer
• mb_parse_str — Parse GET/POST/COOKIE data and set global variable
• mb_preferred_mime_name — Get MIME charset string
• mb_regex_encoding — Returns current encoding for multibyte regex as string
• mb_regex_set_options — Set/Get the default options for mbregex functions
• mb_send_mail — Send encoded mail
• mb_split — Split multibyte string using regular expression
• mb_strcut — Get part of string
• mb_strimwidth — Get truncated string with specified width
• mb_stripos — Finds position of first occurrence of a string within another, case insensitive
• mb_stristr — Finds first occurrence of a string within another, case insensitive
• mb_strlen — Get string length
• mb_strpos — Find position of first occurrence of string in a string
• mb_strrchr — Finds the last occurrence of a character in a string within another
• mb_strrichr — Finds the last occurrence of a character in a string within another, case insensitive
• mb_strripos — Finds position of last occurrence of a string within another, case insensitive
• mb_strrpos — Find position of last occurrence of a string in a string
• mb_strstr — Finds first occurrence of a string within another
• mb_strtolower — Make a string lowercase
• mb_strtoupper — Make a string uppercase
• mb_strwidth — Return width of string
• mb_substitute_character — Set/Get substitution character
• mb_substr_count — Count the number of substring occurrences
• mb_substr — Get part of string

Source:  PHP.net