If you’re using a multi-byte character encoding and do not just want to remove the first four bytes like substr does, use the multi-byte counterpart mb_substr. This does of course will also work with single-byte strings.

If the date is always given in a specific format you could try to convert it to a DateTime object and format the output.

$dateString = '15-Feb-2009 2:24 PM';
$date = DateTime::createFromFormat('j-M-Y g:i A', $dateString);
echo $date->format('Y-m-d G:i'); // will show "2009-02-15 14:24"

In general you should try in avoid holding a date in a string. Convert it to a DateTime -- this makes it also easier for you to manipulate the object (e.g. move date +1 day)

One way would be to explode the string in "words", using explode or preg_split (depending on the complexity of the words separators : are they always one space ? )

For instance :

$string = "R 124 This is my message";
$words = explode(' ', $string);
var_dump($words);

You'd get an array like this one :

array
  0 => string 'R' (length=1)
  1 => string '124' (length=3)
  2 => string 'This' (length=4)
  3 => string 'is' (length=2)
  4 => string 'my' (length=2)
  5 => string 'message' (length=7)

Then, with array_slice, you keep only the words you want (not the first two ones) :

$to_keep = array_slice($words, 2);
var_dump($to_keep);

Which gives :

array
  0 => string 'This' (length=4)
  1 => string 'is' (length=2)
  2 => string 'my' (length=2)
  3 => string 'message' (length=7)

And, finally, you put the pieces together :

$final_string = implode(' ', $to_keep);
var_dump($final_string);

Which gives...

string 'This is my message' (length=18)

And, if necessary, it allows you to do couple of manipulations on the words before joining them back together :-)
Actually, this is the reason why you might choose that solution, which is a bit longer that using only explode and/or preg_split ^^