Check if variable starts with 'http'
if (strpos($source, 'http') === 0) { $source = "<a href=\"$source\">$source</a>";}Note I use ===, not == because strpos returns boolean false if the string does not contain the match. Zero is falsey in PHP, so a strict equality check is necessary to remove ambiguity.
Reference:
You want the substr() function.
if(substr($source, 0, 4) == "http") { $source = "<a href='$source'>$source</a>";}