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I was able to change my PHP version to 7.2, then I ran the install.
When I got to running the install/upgrade.php step in the browser, I got this error message:
Fatal error: Uncaught Symfony\Component\Debug\Exception\ContextErrorException: Warning: Declaration of Cart::removeCartRule($id_cart_rule) should be compatible with CartCore::removeCartRule($id_cart_rule, bool $useOrderPrices = false) in /home/customer/www/mysite.com/public_html/shop/override/classes/Cart.php:0 Stack trace: #0 /home/customer/www/mysite.com/public_html/shop/classes/PrestaShopAutoload.php(152): Symfony\Component\Debug\ErrorHandler->handleError(2, 'Declaration of ...', '/home/customer/...', 0, Array) #1 /home/customer/www/mysite.com/public_html/shop/classes/PrestaShopAutoload.php(152): require_once() #2 [internal function]: PrestaShopAutoload->load('Cart') #3 /home/customer/www/mysite.com/public_html/shop/src/PrestaShopBundle/Install/Upgrade.php(315): spl_autoload_call('Cart') #4 /home/customer/www/mysite.com/public_html/shop/src/PrestaShopBundle/Install/Upgrade.php(814): PrestaShopBundle\Install\Upgrade->initContext() #5 /home/customer/www/mysite.com/public_html/shop/install/upgrade/upgrade.php(93): P in /home/customer/www/mysite.com/public_html/shop/override/classes/Cart.php on line 0
Any ideas why this is happening?