Yeah, this bug exists in phpBB 3.x for a long time, but actually it appears only when running under PHP 5.3.x and newer one.

includes/functions_content.php (get_context() function, line 240)

    // first replace all whitespaces with single spaces    $text = preg_replace('/ +/', ' ', strtr($text, "\t\n\r\x0C ", '     '), $text); 

should be:

    // first replace all whitespaces with single spaces    $text = preg_replace('/ +/', ' ', strtr($text, "\t\n\r\x0C ", '     '), utf8_strlen($text)); 

AFAIK PHP 5.3 has more restrictive functions parameters mechanism, that's why previous PHP versions have never reported about the fact, that a string instead of a long is passed to the function. This issue also makes the text of a post not display in search results, when running under PHP 5.3.