I see a lot of usage in the field where people rely on this function to truncate a string to a given length and append some chars at the end following example #1 in the above documentation.

While this works just fine with Western alphabets, it should be noted that a string's width is NOT necessarily the same as its length.

In Chinese, Japanese and Korean, some characters can be represented as full or half width, which may lead to unexpected results...

<?php
$str = ['English' => 'Switzerland',
        'Half width' => 'ｽｲｽ',
        'Full width' => 'スイス',
       ];
foreach ($str as $w => $s) {
    printf("%-10s: %s (bytes=%d chars=%d width=%d)\nSubstring : %s\nTrim width: %s\n\n",
        $w, $s,
        strlen($s), mb_strlen($s), mb_strwidth($s),
        mb_substr($s, 0, 3),
        mb_strimwidth($s, 0, 3)
    );
}
/* Output
# With ASCII, chars == width, so everything works as expected
English   : Switzerland (bytes=11 chars=11 width=11)
Substring : Swi
Trim width: Swi

# With half-width katakanas, it works too
Half width: ｽｲｽ (bytes=9 chars=3 width=3)
Substring : ｽｲｽ
Trim width: ｽｲｽ

# Full-width katakanas are twice as wide, so we only get the 1st 'su' !
Full width: スイス (bytes=9 chars=3 width=6)
Substring : スイス
Trim width: ス
*/
>?

mb_strimwidth will only append "trim_marker" if the string is over the width.

*test code*
     $HashTags = 'Results';
     $socialmediatext = 'abcdefghijklmnopqrstuvwxyz';
     for( $i=0; $i<=20; $i++ )
     {
         $socialmediatext .= '.';
         $Twittext = mb_strimwidth($socialmediatext,0,35,$HashTags);
         echo "<p>[".strlen($Twittext)."]$Twittext</p>";
     }

*output*
[27]abcdefghijklmnopqrstuvwxyz.
[28]abcdefghijklmnopqrstuvwxyz..
[29]abcdefghijklmnopqrstuvwxyz...
[30]abcdefghijklmnopqrstuvwxyz....
[31]abcdefghijklmnopqrstuvwxyz.....
[32]abcdefghijklmnopqrstuvwxyz......
[33]abcdefghijklmnopqrstuvwxyz.......
[34]abcdefghijklmnopqrstuvwxyz........
[35]abcdefghijklmnopqrstuvwxyz.........
[35]abcdefghijklmnopqrstuvwxyz..Results
[35]abcdefghijklmnopqrstuvwxyz..Results
[35]abcdefghijklmnopqrstuvwxyz..Results
[35]abcdefghijklmnopqrstuvwxyz..Results
[35]abcdefghijklmnopqrstuvwxyz..Results
[35]abcdefghijklmnopqrstuvwxyz..Results
[35]abcdefghijklmnopqrstuvwxyz..Results
[35]abcdefghijklmnopqrstuvwxyz..Results
[35]abcdefghijklmnopqrstuvwxyz..Results
[35]abcdefghijklmnopqrstuvwxyz..Results
[35]abcdefghijklmnopqrstuvwxyz..Results
[35]abcdefghijklmnopqrstuvwxyz..Results