The obvious thing to do is read the documentation.
But to help: substr($str, $start, $end);
$str is your text
$start is the character index to begin at. In your case, it is likely 0 which means the very beginning.
$end is where to truncate at. Suppose you wanted to end at 15 characters, for example. You would write it like this:
<?php
$text = "long text that should be truncated";
echo substr($text, 0, 15);
?>
and you would get this:
long text that
makes sense?
EDIT
The link you gave is a function to find the last white space after chopping text to a desired length so you don't cut off in the middle of a word. However, it is missing one important thing - the desired length to be passed to the function instead of always assuming you want it to be 25 characters. So here's the updated version:
function truncate($text, $chars = 25) {
if (strlen($text) <= $chars) {
return $text;
}
$text = $text." ";
$text = substr($text,0,$chars);
$text = substr($text,0,strrpos($text,' '));
$text = $text."...";
return $text;
}
So in your case you would paste this function into the functions.php file and call it like this in your page:
$post = the_post();
echo truncate($post, 100);
This will chop your post down to the last occurrence of a white space before or equal to 100 characters. Obviously you can pass any number instead of 100. Whatever you need.