如何延迟发送电子邮件？点击按钮立即发送，但要等到下午6点，直到实际发送电子邮件

I have a php script that does some processing (creates remittance advice PDFs, self-billing invoices, Sage CSV file etc...) and at the end outputs a screen with a form, in which the names and e-mail addresses of the people paid appear. User makes a selection of names by clicking check boxes and then there is a Send button which sends out emails with remittance advices and self-billing invoices attached. This all works nicely, BUT they now decided that when they click the Send button, they would like the e-mails to be sent NOT straight-away, but at 6.00pm.I have a php script that does some processing (