How to simulate two date processing functions of SQL Server_PHP tutorial

//It is very inconvenient to process dates in PHP. For example, find the month difference between two dates? What to do?
//File name: date.inc.php3　
//Before using these two functions, you must first convert the date or date time into the timestamp type.
//For example:
//$today=mktime(0,0,0,date("m"),date("d"),date("Y"));

/****Simulate the dateadd function in sqlserver************
$part type: string　
Value range: year, month, day, hour, min, sec　
Represents: which part of the date to be added
$n type: numeric value
Indicates: How much to add, decide which part to add based on $part
Can be negative
$datetime type: timestamp　
Represents: increasing base
Return type: timestamp　
**************Finish**************/　
function dateadd($part,$n,$datetime){
$year=date("Y",$datetime);
$month=date("m",$datetime);
$day=date("d",$datetime);
$hour=date("H",$datetime);
$min=date("i",$datetime);
$sec=date("s",$datetime);
$part=strtolower($part);
$ret=0;　
switch ($part) {　
case "year":　
$year =$n;　
break;　
case "month":　
$month =$n;　
break;　
case "day":　
$day =$n;　
break;　
case "hour":　
$hour =$n;　
break;　
case "min":　
$min =$n;　
break;　
case "sec":　
$sec =$n;　
break;　
default:　
return $ret;　
break;　
}　
$ret=mktime($hour,$min,$sec,$month,$day,$year);
return $ret;　
}　

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