<?php

$sqlReturn = [
				['2017-07-17 06:00:00', '100'],
				['2017-07-17 06:05:00', '280'],
				['2017-07-17 06:30:00', '70'],
				['2017-07-17 06:35:00', '80'],
				['2017-07-17 07:45:00', '80'],
				['2017-07-17 07:50:00', '80'],
				['2017-07-19 07:50:00', '80'],	
				['2017-07-19 07:55:00', '80'],	
             ];
		
		
$downtimes = [];

for ($i = 0; $i < count($sqlReturn)-1; $i++) {

	$lowTime = $sqlReturn[$i][0];
	$highTime = $sqlReturn[$i+1][0];
	
	$start = new DateTime($lowTime);
	$finish = new DateTime($highTime);

	$diffInMins = abs($finish->getTimestamp() - $start->getTimestamp()) / 60;


	if ($diffInMins != 5) {
		$temp = ['Интервал' => $lowTime . ' - ' . $highTime, 
		         'Время проеба' =>  $diffInMins . ' минут'];
		$downtimes[] = $temp;
	}
}

print_r($downtimes);