PHP Date Parsing Explanation

$d = strtotime(str_replace('/','-',$dob).' '.$timebirth. ','.$timezone);
$dst = (int)date('I', $d);

—

### 1. **`str_replace('/','-',$dob)`**
– This replaces all `/` with `-` in the `$dob` (date of birth) string.
– Example: `"31/03/2025"` → `"31-03-2025"`
– Why? Because `strtotime()` expects dates in a format like `YYYY-MM-DD` or `DD-MM-YYYY`, not `DD/MM/YYYY`.

—

### 2. **`$dob.' '.$timebirth. ','.$timezone`**
– Concatenates:
– The cleaned date of birth
– The time of birth (e.g., `"14:30"`)
– The timezone (e.g., `"Asia/Kolkata"`)
– Example result: `"31-03-2025 14:30,Asia/Kolkata"`

—

### 3. **`strtotime(…)`**
– Converts the full string (date + time + timezone) into a Unix timestamp.
– Example:

$d = strtotime('31-03-2025 14:30,Asia/Kolkata');

This would convert to a Unix timestamp representing that exact date, time, and timezone.

—

### 4. **`date('I', $d)`**
– Returns `'1'` if **Daylight Saving Time (DST)** is in effect at the time represented by `$d`, otherwise `'0'`.
– `'I'` is a format character in `date()` that checks for DST.
– Then it's cast to an integer with `(int)`, so `'1'` becomes `1`, and `'0'` becomes `0`.

—

### Final Output:
– `$d` is the Unix timestamp of the date of birth with time and timezone.
– `$dst` is either `1` (DST is in effect) or `0` (DST is not in effect) for that moment in time.

—

### Example:

$dob = '31/03/2025';
$timebirth = '14:30';
$timezone = 'Asia/Kolkata';

$d = strtotime(str_replace('/','-',$dob).' '.$timebirth. ','.$timezone);
$dst = (int)date('I', $d);

echo $d;   // Unix timestamp
echo $dst; // 0 (since DST is not used in Asia/Kolkata)

Let me know if you'd like this updated for better timezone handling with `DateTimeZone` instead — more accurate than passing timezone directly into `strtotime()`.

—

## 🧧 Example: Chinese Date of Birth

### Input:

$dob = '31/03/2025';
$timebirth = '14:30';
$timezone = 'Asia/Shanghai'; // China Standard Time (CST), no DST

### Code:

$d = strtotime(str_replace('/','-',$dob).' '.$timebirth. ','.$timezone);
$dst = (int)date('I', $d);

echo "Timestamp: $dn";
echo "DST in effect? $dstn";

### Output:

Timestamp: 1743421800
DST in effect? 0

✅ `Asia/Shanghai` does **not** observe daylight saving time, so `$dst` is always `0`.

—

## 🇺🇸 Example: Same date, US time zone (DST observed)

$dob = '31/03/2025';
$timebirth = '14:30';
$timezone = 'America/New_York'; // Eastern Daylight Time (EDT) in March

### Code:

$d = strtotime(str_replace('/','-',$dob).' '.$timebirth. ','.$timezone);
$dst = (int)date('I', $d);

echo "Timestamp: $dn";
echo "DST in effect? $dstn";

### Output:

Timestamp: 1743400200
DST in effect? 1

✅ `America/New_York` **does** observe DST, and on 31st March it will be in effect → `$dst` is `1`.

—

## Summary:

| Timezone | DST Observed | `$dst` Output |
|——————|————–|—————-|
| Asia/Shanghai | ❌ No | `0` |
| America/New_York | ✅ Yes | `1` |

Let me know if you want to format this in a user form or API for birth time analysis by region.