PHP getting full image attribute

I am fetching url like facebook. For all images in that page I get them with

$src = $tag->getAttribute('src');
 

it works fine if the foreign site put a full webpath to their images. But sometimes they use relative path only and that returns a broken image.

Since I have the full url, is there a way to check if $src contains http:// or https?

if not then I could do

$src = $link . '/' . $src; 
 

for those relative images. I tried, they display. But if they already had it, then they will not display as path then contains double url.

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