$startpos adjustment may return minus value.

[infinitail]

In extractRelevant() function $startpos variable is adjusted for keep text length. But some case like below, it may return minus value and bogus hilighted text.

$textlength = 323
$startpos = 95
$locations = 145
$rellength = 300

adjusted $startpos = 95 - (323 - 95) / 2 = -19

I suggest add max() for fix this issue.

        if ($textlength - $startpos < $rellength) {
            $startpos = $startpos - ($textlength - $startpos) / 2;
+           $startpos = max($startpos, 0);
        }

[nticaric]

ok, please send a pull request