Significantly updated version (with new $pat4 utilising \R properly, its results and comments):
Note that there are (sometimes difficult to grasp at first glance) nuances of meaning and application of escape sequences like \r, \R and \v - none of them is perfect in all situations, but they are quite useful nevertheless. Some official PCRE control options and their changes come in handy too - unfortunately neither (*ANYCRLF), (*ANY) nor (*CRLF) is documented here on php.net at the moment (although they seem to be available for over 10 years and 5 months now), but they are described on Wikipedia ("Newline/linebreak options" at https://en.wikipedia.org/wiki/Perl_Compatible_Regular_Expressions) and official PCRE library site ("Newline convention" at http://www.pcre.org/original/doc/html/pcresyntax.html#SEC17) pretty well. The functionality of \R appears somehow disappointing (with default configuration of compile time option) according to php.net as well as official description ("Newline sequences" at https://www.pcre.org/original/doc/html/pcrepattern.html#newlineseq) when used improperly.
A hint for those of you who are trying to fight off (or work around at least) the problem of matching a pattern correctly at the end ($) of any line in multiple lines mode (/m).
<?php 
// Various OS-es have various end line (a.k.a line break) chars:
// - Windows uses CR+LF (\r\n);
// - Linux LF (\n);
// - OSX CR (\r).
// And that's why single dollar meta assertion ($) sometimes fails with multiline modifier (/m) mode - possible bug in PHP 5.3.8 or just a "feature"(?).
$str="ABC ABC\n\n123 123\r\ndef def\rnop nop\r\n890 890\nQRS QRS\r\r~-_ ~-_";
//          C          3                   p          0                   _
$pat1='/\w$/mi';    // This works excellent in JavaScript (Firefox 7.0.1+)
$pat2='/\w\r?$/mi';    // Slightly better
$pat3='/\w\R?$/mi';    // Somehow disappointing according to php.net and pcre.org when used improperly
$pat4='/\w(?=\R)/i';    // Much better with allowed lookahead assertion (just to detect without capture) without multiline (/m) mode; note that with alternative for end of string ((?=\R|$)) it would grab all 7 elements as expected
$pat5='/\w\v?$/mi';
$pat6='/(*ANYCRLF)\w$/mi';    // Excellent but undocumented on php.net at the moment (described on pcre.org and en.wikipedia.org)
$n=preg_match_all($pat1, $str, $m1);
$o=preg_match_all($pat2, $str, $m2);
$p=preg_match_all($pat3, $str, $m3);
$r=preg_match_all($pat4, $str, $m4);
$s=preg_match_all($pat5, $str, $m5);
$t=preg_match_all($pat6, $str, $m6);
echo $str."\n1 !!! $pat1 ($n): ".print_r($m1[0], true)
    ."\n2 !!! $pat2 ($o): ".print_r($m2[0], true)
    ."\n3 !!! $pat3 ($p): ".print_r($m3[0], true)
    ."\n4 !!! $pat4 ($r): ".print_r($m4[0], true)
    ."\n5 !!! $pat5 ($s): ".print_r($m5[0], true)
    ."\n6 !!! $pat6 ($t): ".print_r($m6[0], true);
// Note the difference among the three very helpful escape sequences in $pat2 (\r), $pat3 and $pat4 (\R), $pat5 (\v) and altered newline option in $pat6 ((*ANYCRLF)) - for some applications at least.

/* The code above results in the following output:
ABC ABC

123 123
def def
nop nop
890 890
QRS QRS

~-_ ~-_
1 !!! /\w$/mi (3): Array
(
    [0] => C
    [1] => 0
    [2] => _
)

2 !!! /\w\r?$/mi (5): Array
(
    [0] => C
    [1] => 3
    [2] => p
    [3] => 0
    [4] => _
)

3 !!! /\w\R?$/mi (5): Array
(
    [0] => C

    [1] => 3
    [2] => p
    [3] => 0
    [4] => _
)

4 !!! /\w(?=\R)/i (6): Array
(
    [0] => C
    [1] => 3
    [2] => f
    [3] => p
    [4] => 0
    [5] => S
)

5 !!! /\w\v?$/mi (5): Array
(
    [0] => C

    [1] => 3
    [2] => p
    [3] => 0
    [4] => _
)

6 !!! /(*ANYCRLF)\w$/mi (7): Array
(
    [0] => C
    [1] => 3
    [2] => f
    [3] => p
    [4] => 0
    [5] => S
    [6] => _
)
 */
?>
Unfortunately, I haven't got any access to a server with the latest PHP version - my local PHP is 5.3.8 and my public host's PHP is version 5.2.17.