How to detect padding on an integer and treat it as a string?

I have this function to prepare variable to be used in a SQL query:

function sqlize($mInput)
{
    if (!isset($mInput))
        $mInput = "null";

    elseif (strtolower($mInput) == "null") { }

    elseif (is_numeric($mInput)) { }

    elseif (is_string($mInput))
    {
        $mInput = trim($mInput);
        $mInput = addslashes($mInput);
        $mInput = '"' . $mInput . '"';
    }
    else
        $mInput = "null";
    return $mInput;
}

I have a string "0004", which is going in a "varchar field", is cought by is_numeric, and is saved as "4" and not "0004". Is there a way to detect the padding and process it as a string?

Thank you!

EDIT

Correct answer based on Cesar and webbiedave tips:

    function sqlize($mInp开发者_如何学Cut)
    {
        switch(TRUE)
      {
            default: break;
            case (!isset($mInput)): $mInput = "null"; break;
            case (strtolower($mInput) == "null"): break;
            case (is_numeric($mInput)):
                if ((string) intval($mInput) === (string) $mInput)
                    break;
            case (is_string($mInput)):
                    $mInput = trim($mInput);
                    #v2
                    $mInput = $_SESSION['oCore']->oDatabase->sanatize($mInput);
// ADOdb's oDb->qstr();                 
                    # v1
                    #$mInput = addslashes($mInput);
                    #$mInput = '"' . $mInput . '"';
                    break;
        }
        return $mInput;
}

instead of is_numeric() use (int)$var == $var
I saw a beautiful use of the switch, should be something like this:

function sqlize($mInput)
{
    switch(TRUE)
    {
        default:
        case (!isset($mInput)): $mInput = "null"; break;
        case (strtolower($mInput) == "null"): break;
        case (is_numeric($mInput)): break;
        case (is_string($mInput)):
            $mInput = trim($mInput);
            $mInput = addslashes($mInput);
            $mInput = '"' . $mInput . '"';
            break;
    }
    return $mInput;
}