如何在PHP中将日期变量和时间变量转换为一个时间戳变量？

I have a variable (startsAt) that is a date variable, and I have a variable (startsTime) that is a time variable. I would like to combine those two variables into a timestamp and use that timestamp to update a database in SQL. Can anybody help with this?I have a variable (startsAt) that is a date var