The confusing strtotime() function in PHP

For upon |Frequently PHP programmers get confused about the use of the I month, -1 month, and next month in strtotime () function. and hence it leaves some impression on the programmer that this function is not that reliable.

Let’s take one example of strtotime call with -1 month and see why it leaves this impression.

date(“Y-m-d”,strtotime(“-1 month”))  // Assume today is 2018-07-31

What’s the output of the above call? The answer is 2018-07-01. Why not 2018-06-30? So people get confused. It appears that this is wrong at first glance, but if think again, this output is reasonable.

Below is how the logic works

• It first minus one month, so the date becomes 2017-06-31
• Since June does not have 31 days, hence it will be canonicalized and becomes 2018-01-01. This is similar to why 2:60 will be 3:00

Now you get the point. This logic can also be verified with the below call.

var_dump(date(“Y-m-d”, strtotime(“2018-06-31”)));

The output will be 2018-07-01. This behavior also applies to other months which have different days. Below are some more examples:

So how to make it work as expected? Starting from PHP 5.3, strtotime() added a few new tokens which can be used to correct this. They are the first day and last day of the.

What if people are still using the old version of PHP? Mktime() could be used to achieve similar results.

Hence don’t get panic when see this kind of “unexpected” behavior in the future.

Reference: http://www.laruence.com/2018/07/31/3207.html

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