<?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}"; 

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