How to use PHP elseif [duplicate]

I am trying to format the user's website. The If and Else Statements work, but the Else if Statements do not work. What am I doing wrong? Thank you. I appreciate your help.

$website = formatWebsite(($_POST['website'])); 

function formatWebsite($website)
{
    //Checks to see if website has security 
    $scheme = substr($website, 0, 9); 
    if ($scheme =  "https://www")
    {
        $scheme = substr($website, 0, 7); 
        return $website; 
    }
    elseif($scheme = "https://")
    {
        $formattedWebsite = "https://www". trim($website, "https://");  
        $scheme = substr($website, 0, 6); 
        return $formattedWebsite; 
    }
    
    //Removes http:// from user's website
    elseif ($scheme = "http://")
    {
        $formattedWebsite = trim($website, "http://");  
        $scheme = substr($website, 0, 2); 
        return $formattedWebsite; 
        
    }
    //Makes sure www. is in the user's website
    elseif ($scheme != "www")
    {
        $formattedWebsite = "www.". $website; 
        return $formattedWebsite; 
    }
    //If the user's website has www. print the websites 
    else 
    {
        return $website; 
    }

}