试试这个方法：

$interval = "15 minutes";
...
$current    = time();
add_time   = strtotime($current.'+'.$interval);

你也可以给我们更多的信息，最终错误抛出。最终启用调试：

ini_set('display_errors', 1);
ini_set('display_startup_errors', 1);
error_reporting(E_ALL);

更新1
在foreach中，你试图修改你正在循环的同一个数组（$times [$key]），似乎没有意义。

看看这个我希望我已经正确理解了您的问题并提供了所需的解决方案。

<?php
$startTime     = '09:00';
$endTime       = '17:00';
$interval      = 15; // minutes
$weekNumber    = date('w');
$excludedSlots = [
    ['day' => '4', 'start' => '10:00', 'end' => '12:00'],
    ['day' => '4', 'start' => '12:30', 'end' => '13:30'],
    ['day' => '5', 'start' => '15:30', 'end' => '16:00']
];

$timeRange = generateTimeRange($startTime, $endTime, $interval, $weekNumber, $excludedSlots);
echo $timeRange;

function generateTimeRange($startTime, $endTime, $interval, $weekNumber, $excludedSlots) {
    $timeRange = '';
    $currentTime = $startTime;

    while ($currentTime <= $endTime) {
        $isExcluded = false;

        foreach ($excludedSlots as $excludedSlot) {
            if ($weekNumber == $excludedSlot['day'] && $currentTime >= $excludedSlot['start'] && $currentTime < $excludedSlot['end']) {
                $isExcluded = true;
                break;
            }
        }

        if (!$isExcluded) {
            $timeRange .= "<option value='{$currentTime}'>{$currentTime}</option>";
        } else {
            $timeRange .= "<option value='{$currentTime}' disabled>{$currentTime} (Blocked)</option>";
        }

        $currentTime = date('H:i', strtotime($currentTime) + ($interval * 60));
    }

    return $timeRange;
}
?>

下面是在本地机器上测试的更新代码

<?php

$open_from = "09:00";
$open_to = "17:00";
$interval = "15 mins";  

$today_number = date('w');

$intervals = array(
    array(
        'day' => '4',
        'start' => '10:00',
        'end' => '12:00',
    ),
    array(
        'day' => '4',
        'start' => '12:30',
        'end' => '13:30',
    ),
    array(
        'day' => '5',
        'start' => '15:30',
        'end' => '16:00',
    ),
);  

if(!isset($_GET['date'])){                  
    $date = date('d-m-Y');          
}else{          
    $date = $_GET['date'];          
} 

$times = create_time_range($open_from, $open_to, $interval);

foreach ($times as $key => $time) { 
    $day = date('w', strtotime($date));
    $timeFrame = date('H:i', $time);
   
    foreach ($intervals as $slot) {
        if ($day == $slot['day'] && $timeFrame >= $slot['start'] && $timeFrame < $slot['end']) {
            echo 'No </br>';
        } else {
            echo 'Yes </br>';
        }
    }
}

function create_time_range($start, $end, $interval) { 

  $start_time = strtotime($start); 
  $end_time   = strtotime($end); 

  $current    = time(); 
  $add_time   = strtotime('+'.$interval, $current); 

  $diff       = $add_time-$current; 

  $times = array(); 
  while ($start_time < $end_time) { 
      $times[] = $start_time; 
      $start_time += $diff; 
  } 
  $times[] = $start_time; 
  return $times; 
  
}

为了回答您的问题，您可以使用以下代码来实现您想要的结果：

if(!isset($_GET['date']))
    $date = date('d-m-Y');
else
    $date = $_GET['date'];

$startTime     = '09:00';
$endTime       = '17:00';
$interval      = 15; // minutes
$day           = date('N', strtotime($date));

$bookedSlots = array(
    array(
        'day' => '4',
        'start' => '10:00',
        'end' => '12:00',
    ),
    array(
        'day' => '4',
        'start' => '12:30',
        'end' => '13:30',
    ),
    array(
        'day' => '5',
        'start' => '15:30',
        'end' => '16:00',
    ),
);

function fetchTimeIntervals($startTime, $endTime, $intervalMinutes) {
    global $day,$bookedSlots;

    $htmlContent = '';

    // Convert start and end times to DateTime objects
    $start  = new DateTime($startTime);
    $end    = new DateTime($endTime);

    // Create an interval object based on the specified minutes
    $interval = DateInterval::createFromDateString($intervalMinutes.' minutes');

    // Create a date period object based on the start, end, and interval
    $period = new DatePeriod($start, $interval, $end);

    // Iterate through each interval and check each slot whether it is booked or not
    foreach ($period as $slot) {
        foreach ($bookedSlots as $subArr) {
            $isSlotBooked = false;

            $bookedSlotStartTime = new DateTime($subArr['start']);
            $bookedSlotEndTime   = new DateTime($subArr['end']);

            if($subArr['day'] == $day && $bookedSlotStartTime->getTimestamp()-$slot->getTimestamp() <= 0 && $bookedSlotEndTime->getTimestamp()-$slot->getTimestamp() > 0)
                $isSlotBooked = true;
        }

        $disableClass = ($isSlotBooked) ? 'disabled' : '';
        $slotTime     = $slot->format("H:i");
        $htmlContent .= "<option value='{$slotTime}' {$disableClass}>{$slotTime}</option>";
    }
    return $htmlContent;
}

echo $timeIntervals = fetchTimeIntervals($startTime, $endTime, $intervalMinutes);
?>