Dynamic setter/getter using __call magic method of php

DynamicSetterGetter.php

<?php
class Person
{
    protected $firstName;
    protected $lastName;

    /**
     * Magic method call for setter and getter
     * 
     * @param $methodName
     * @param $params
     * 
     * @return mixed
     */
    public function __call($methodName, $params = null)
    {
        // Check if a property does exists
        if (!property_exists($this, $attr)) {
		    exit('Opps! Property does not exists');
	    }
	    
	    // Get the method prefix to determine action
        $methodPrefix = substr($methodName, 0, 3);
        // Get the attribute to do action with
        $attr = self::camelCase(substr($methodName, 3));

        // Check action (Set/Get)
        if ($methodPrefix == 'set' && count($params) == 1) {
            // Get the parameter value
            $value = $params[0];
            // Set the value
            $this->{$attr} = $value;
        } elseif ($methodPrefix == 'get') {
            // Return the value
            return $this->{$attr};
        } else {
            exit('Opps! The method is not defined!');
        }
    }

    /**
     * Convert a string into a CamelCase string format
     * 
     * @param $str
     * @param $noStrip
     * 
     * @return String
     */
    public static function camelCase($str, array $noStrip = array())
    {
        // non-alpha and non-numeric characters become spaces
        $str = preg_replace('/[^a-z0-9' . implode("", $noStrip) . ']+/i', ' ', $str);
        $str = trim($str);
        // uppercase the first character of each word
        $str = ucwords($str);
        $str = str_replace(" ", "", $str);
        $str = lcfirst($str);
 
        return $str;
    }
}

$person = new Person();
$person->setFirstName('alleo');
echo $person->getFirstName();

There is a small error in the above code. Line 17-21, need to be moved after line 26