<?php

$horas = array(
    '-05:00:00',
    '08:00:00'
);

$seconds = 0;

foreach ( $horas as $hora )
{
    list( $g, $i, $s ) = explode( ':', $hora );
    if ($g < 0) {
    	$i *= -1;
    	$s *= -1;
    }
    $seconds += $g * 3600;
    $seconds += $i * 60;
    $seconds += $s;
}

$hours    = floor( $seconds / 3600 );
$seconds -= $hours * 3600;
$minutes  = floor( $seconds / 60 );
$seconds -= $minutes * 60;

echo "{$hours}:{$minutes}:{$seconds}";