sed "s|http://.*|http://$URL|"

URL="1.1.1.1/login.php?user=admin\&pass=password"

echo "http://something" | sed -e "s|http://.*|http://$URL|"

http://1.1.1.1/login.php?user=admin&pass=password

URL="1.1.1.1/login.php?user=admin&pass=password"
awk -vurl="$URL" '/http:///{  gsub("http://" , "http://"url) }1' file

$ URL="1.1.1.1/login.php?user=admin&pass=password"
$ echo "http://abcd" | sed -e "|http://.*|c$URL"
1.1.1.1/login.php?user=admin&pass=password

URL="1.1.1.1/login.php?user=admin&pass=password"
URL=$(echo "$URL" | sed 's/&/\&/')    # substitute to escape the ampersand
echo "$OTHER" | sed "s|http://.*|http://$URL|"