Keep getting "undefined variable: measurements" error.

private $measurements = array(
    "cup",
    "tsp",
    "tbsp",
    "mg",
    "g",
    "kg",
    "ml",
    "l"
    );

//print (echo) the recipe to screen
public function displayRecipe()
{
    return $this->title . " by " . $this->source . "\n";
}

// set the recipe title attribute to camel case
public function setTitle($title)
{
    $this->title = ucwords($title);
}

public function getTitle()
{
    return $this->title;
}
// set the recipe source attribute to camel case
public function setSource($source)
{
    $this->source = ucwords($source);
}

public function getSource()
{
    return $this->source;
}

//set ingredients via associative array
public function addIngredient($item, $amount = null, $measure = null)
{
    //check to ensure only valid measurements entered
    if($measure != null && !in_array(strtolower($measure), $this->$measurements))
    {
        exit("Measure must be valid (metric) measurement.  Use one of the following: " . implode(", ", $this->measurements) . "\n");
    }

    //check to ensure only int or float entered for amount
    if($amount != null && !is_float($amount) && !is_int($amount))
    {
        exit("The amount must be a decimal. " . gettype($amount) . " entered.  Please try again.\n");
    }

    $this->ingredients[] = array(
        "item"=>ucwords($item),
        "amount"=>$amount,
        "measure"=>strtolower($measure)
        );
}