Could you please provide short example of any of these functions working wrong - as a short isolated script, not as a part of complicated code?

I already simplified it down and added a bunch of echo's so you can see the walk through.

1) Place the sample input into a file called 'mime.txt'.
2) Place the php script into a file in the same directory.
3) Open the example script. I put a simple call in it already.

The events are pretty straight forward:
1) Open a file
2) Read a line into an array of tokens
3) skip the first token (it is the mime type)
4) check to see if the pased in arg matches any of the other tokens
5) return the first token from a line with a matching string or default to "application/binary"

The extra comments in the script are for other regular expressions that SHOULD work but do not.

Sorry I spotted a bug in the sample call. It should be:

echo("<br><H1>Mime type for .H is " . get_mime_type("H") . "</H1>");

The real problem here is that if you look at one sample input:

"text/html               htm HTM html HTML shtml SHTML"

Which in regular expression land could look like this:
text/html\t\shtm\sHTM\shtml\sHTML\sshtml\sSHTML\n

when parsed with explode("[\t\s]") or the same without the regex escapes (as in the current example script) an array of 7 elements should be created:
0=>"text/html"
1=> "htm"
2=> "HTM"
3=> "html"
4=> "HTML"
5=> "shtml"
6=> "SHMTL"

But instead (as shown in the sample output) an array of 6 elements is being created:
0=> "htm"
1=> "HTM"
2=> "html"
3=> "HTML"
4=> "shtml"
5=> "SHMTL"

Additionally ALL of the regular expresion character classes [:space:] and escapes '\t\n\s' are not functioning correctly. I have tried rebuilding php with perl regex, system and php regex support without any of them working.

Having written a few regex engines myself I know how hard it is to get it right. But this is really a huge bug in explode and strtok.

I'm a little confused by your examples. First, explode does not accept regular expression argument. It requires string separator argument. Second, split, which does expect regular expression, does not know \s escape - that's Perl escape, and you should use preg_split for it or use [:space:] character class.

When I tried split("[\t ]",...) or preg_split("/[\t\s]/", ...) it worked flawlessly for me.

Please provide example of not working  code along the following lines:

===example start
<?
$str = "MY String";
$array = split("pattern",$str);
var_dump($array);
?>

Should be: [0]=>"MY",[1]=>"string", but prints [0]=>"Y s",[1]=>"tring"
===example end

OK, sorry for giving too large a sample program.

Try this:
<?
$str = "   MY    String           is                            hosed    ";
$array = split("[ \t]*",$str);
$len = count($array);
echo("String has: $len tokens\n");
for( $x = 0 ; $x < $len ; $x++ ) {
    print " \$array[$x] => \"$array[$x]\"\n";
}
?>

It returns this:
Warning:  bad regular expression for split() in split.php on line 3
String has: 1 tokens
 $array[0] => ""

When it should return:
String has: 4 tokens
 $array[0] => "MY"
 $array[1] => "String"
 $array[2] => "is"
 $array[3] => "hosed"

Replacing split("[ \t]*",$str); with split("[[:space:]]*",$str); returns the same error.

There *does* appear to be a bug here, but it is not what
you seem to think it is. Using your most recent example,
you should have:

String has: 6 tokens
 $array[0] => ""
 $array[1] => "MY"
 $array[2] => "String"
 $array[3] => "is"
 $array[4] => "hosed"
 $array[5] => ""

That's what you asked for.
The bug has something to do with *. Replacing the * in your
example with + gets rid of the error, but is obviously
not quite the same.

OK, the problem is that your regular expression allows null
matches (i.e., [ \t]* matches empty string). That generally
means that delimiter can be empty - which is wrong, because
delimiter never can be empty - it's meaningless. So actually
split is right to say your regexp is bad. Use + instead of *.