Php wait 5 seconds before executing an action

Question

I am working on a PHP script that is used to create a list of products. 

My script:

script.php?start=0&end=500&indexOfFile=0 ->> make a product0.txt file with first 500 products


script.php?start=501&end=1000&indexOfFile=1 ->> product1.txt file with another 500 products


script.php?start=1001&end=1500&indexOfFile=2 ->> product2.txt file with last 500 products

I want to make changes to this script so that it will create all the files automatically. 

When I click on the button I want to:

Make the product0.txt file with the first 500 products.

Wait 5 seconds 

Make the product1.txt file with another 500 products.

Wait 5 seconds

Make the product2.txt file with the last 500 products.

Can someone help me do this?