Simplest way to get a complete list of all the UTF-8 whitespace characters in PHP
This email (archived here) contains a list of all Unicode whitespace characters encoded in UTF-8, UTF-16, and HTML.
In the archived link look for the 'utf8_whitespace_table' function.
static $whitespace = array(
"SPACE" => "\x20",
"NO-BREAK SPACE" => "\xc2\xa0",
"OGHAM SPACE MARK" => "\xe1\x9a\x80",
"EN QUAD" => "\xe2\x80\x80",
"EM QUAD" => "\xe2\x80\x81",
"EN SPACE" => "\xe2\x80\x82",
"EM SPACE" => "\xe2\x80\x83",
"THREE-PER-EM SPACE" => "\xe2\x80\x84",
"FOUR-PER-EM SPACE" => "\xe2\x80\x85",
"SIX-PER-EM SPACE" => "\xe2\x80\x86",
"FIGURE SPACE" => "\xe2\x80\x87",
"PUNCTUATION SPACE" => "\xe2\x80\x88",
"THIN SPACE" => "\xe2\x80\x89",
"HAIR SPACE" => "\xe2\x80\x8a",
"ZERO WIDTH SPACE" => "\xe2\x80\x8b",
"NARROW NO-BREAK SPACE" => "\xe2\x80\xaf",
"MEDIUM MATHEMATICAL SPACE" => "\xe2\x81\x9f",
"IDEOGRAPHIC SPACE" => "\xe3\x80\x80",
);
Years later, this question still has top results on Google when looking for unicode whitespace characters. devio's answer is great, but incomplete. As of this writing (October 2017) Wikipedia has a list of whitespace characters here: https://en.wikipedia.org/wiki/Whitespace_character
This list has specifies 25 code points, whereas the currently accepted answer lists 18. Including the seven other code points, the list is:
U+0009 character tabulation
U+000A line feed
U+000B line tabulation
U+000C form feed
U+000D carriage return
U+0020 space
U+0085 next line
U+00A0 no-break space
U+1680 ogham space mark
U+180E mongolian vowel separator
U+2000 en quad
U+2001 em quad
U+2002 en space
U+2003 em space
U+2004 three-per-em space
U+2005 four-per-em space
U+2006 six-per-em space
U+2007 figure space
U+2008 punctuation space
U+2009 thin space
U+200A hair space
U+200B zero width space
U+200C zero width non-joiner
U+200D zero width joiner
U+2028 line separator
U+2029 paragraph separator
U+202F narrow no-break space
U+205F medium mathematical space
U+2060 word joiner
U+3000 ideographic space
U+FEFF zero width non-breaking space
http://en.wikipedia.org/wiki/Space_%28punctuation%29#Spaces_in_Unicode
Unfortunately, it doesn't give UTF-8, but it does have the character in the web page, so you could cut and paste into your editor (if it saves in UTF-8). Alternatively, http://www.fileformat.info/info/unicode/char/180E/index.htm gives UTF-8 (replace "180E" with the hex UTF-16 value you are looking up).
This also gives a couple extra characters that @devio's excellent answer misses.