Add date in php gives different results - php

Question

I have to get a date which is 6 months added with specific date.
I used the following code
$start_date = "2016-08-30";
$end_date= date( "Y-m-d", strtotime( "$start_date +6 months" ) );
echo $end_date;
which gave result as 2017-03-02
Then I changed the start date in code as below
$start_date = "2016-09-01";
$end_date= date( "Y-m-d", strtotime( "$start_date +6 months" ) );
echo $end_date;
which is giving result as 2017-03-01
Why is this happening at first place? Is there anything wrong with my code?
Using Mysql query
SELECT DATE_ADD('2016-08-30', INTERVAL 6 MONTH)
gives result 2017-02-28
Which is the right solution to get the correct date?

Answer 1

This happens due to PHP's behavior. In this case 6 months are added which gives february(it has 28 days) so it adds three more days , but in MySQL it adds only months.
To solve this use last day of 6 month or last day of sixth month instead of +6 months
Code
$date = new DateTime( '2016-08-30' );
echo $date->format( 'Y-m-d' ), "\n";
$date->modify( 'last day of sixth month' );
echo $date->format( 'Y-m-d' ), "\n";
Code Demo
Output
2016-08-30
2017-02-28
Another Solution
$date = new DateTime( '2016-08-25' );
echo $date->format('Y-m-d'),"\n";
$day = $date->format('j');
$date->modify('first day of sixth month');
$date->modify('+' . (min($day, $date->format('t')) - 1) . ' days');
echo $date->format('Y-m-d');
Code Demo
Output
2016-08-25
2017-02-25
Also refer Relative Formats

Answer 2

This function DOES NOT work from left-to-right as one would think. This function parses the string as a whole, then applies the intervals by size (year, month, ...). Take the following example:
<?php
$Date = strtotime('2011-02-22'); // February 22nd, 2011. 28 days in this month, 29 next year.
echo date('n/j/Y', strtotime('+1 year, +7 days', $Date)); // add 1 year and 7 days. prints 2/29/2012
echo "<br />";
echo date('n/j/Y', strtotime('+7 days, +1 year', $Date)); // add 7 days and 1 year, but this also prints 2/29/2012
echo "<br />";
echo date('n/j/Y', strtotime('+1 year', strtotime('+7 days', $Date))); // this prints 3/1/2012, what the 2nd would do if it was left-to-right
?>