Here's a function that will trim a $string down to a certian number of words, and add a...   on the end of it.
(explansion of muz1's 1st 100 words code)

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<?php
function trim_text($text, $count){
$text = str_replace("  ", " ", $text);
$string = explode(" ", $text);
for ( $wordCounter = 0; $wordCounter <= $count;wordCounter++ ){ 
$trimed .= $string[$wordCounter];
if ( $wordCounter < $count ){ $trimed .= " "; }
else { $trimed .= "..."; }
}
$trimed = trim($trimed);
return $trimed;
}
?>

Usage
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<?php
$string = "one two three four";
echo trim_text($string, 3);
?>

returns:
one two three...