How can I get the last 7 characters of a PHP string?

umh.. like that?

$newstring = substr($dynamicstring, -7);

Safer results for working with multibyte character codes, allways use mb_substr instead substr. Example for utf-8:

$str = 'Ne zaman seni düşünsem';
echo substr( $str, -7 ) . ' <strong>is not equal to</strong> ' .
  mb_substr( $str, -7, null, 'UTF-8') ;

Use substr() with a negative number for the 2nd argument.

$newstring = substr($dynamicstring, -7);

From the php docs:

string substr ( string $string , int $start [, int $length ] )

If start is negative, the returned string will start at the start'th character from the end of string.