How to simulate two SQL Server date processing functions

// It is inconvenient to process dates in PHP, for example, to find the month with a different date? What should I do?
// File name: date. inc. php3
// Before using these two functions, convert the date or date to 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
Indicates the part of the date to be added.
$ N type: value
Indicates the amount to be added. You can decide which part to add based on $ part.
Negative
$ Datetime type: timestamp
Indicates the base number to be added.
Return type: timestamp
**************/
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;
}