Click Settings app.
iPhone settings are displayed. Scroll down the page till you see Safari.
Safari settings are displayed. Click Open Links under General.
Open Links settings are displayed. As you can see, the tick mark is in front of 'In New Tab'. Click 'In Background'.
The tick mark is now in front of 'In Background'. Click Safari on top left to take you to the previous page.
As you can see, Open Links now shows 'In Background'. Now go to Safari browser, open some website and long press a link.
A menu pops up in which the second button says 'Open in Background'. On clicking this button, the new link opens in the background and does not take you away from the current page that you are navigating.