How do you pull first 100 characters of a string in PHP
$small = substr($big, 0, 100);
For String Manipulation here is a page with a lot of function that might help you in your future work.
You could use substr, I guess:
$string2 = substr($string1, 0, 100);
or mb_substr for multi-byte strings:
$string2 = mb_substr($string1, 0, 100);
You could create a function wich uses this function and appends for instance '...'
to indicate that it was shortened. (I guess there's allready a hundred similar replies when this is posted...)
A late but useful answer, PHP has a function specifically for this purpose.
mb_strimwidth
$string = mb_strimwidth($string, 0, 100);
$string = mb_strimwidth($string, 0, 97, '...'); //optional characters for end