Shows the difference between static and dynamic class properties.

not-static-but-static.php

<?php

namespace Static_Or_Dynamic;

/**
 * a simple property list
 *
 */
class Settings_List {

	protected $properties = array();

	public function set( $key, $value ) {

		$this->properties[ $key ] = $value;
	}

	public function get( $key ) {

		return isset( $this->properties[ $key ] )
			? $this->properties[ $key ]
			: NULL;
	}
}

class My_Object {


	/**
	 * @type \Static_Or_Dynamic\List
	 */
	protected $settings;

	public function __construct( Settings_List $settings ) {

		$this->settings = $settings;
	}

	/**
	 * change the memory limit
	 *
	 * @param string $limit
	 */
	public function set_memory_limit( $limit ) {

		$this->settings->set( 'memory_limit', $limit );
	}

	/**
	 * get memory limit
	 *
	 * @return string
	 */
	public function get_memory_limit() {

		return $this->settings->get( 'memory_limit' );
	}
}

$settings = new Settings_List;
$settings->set( 'memory_limit', '40M' );

$cool_feature = new My_Object( $settings );
$better_feature = new My_Object( $settings );

echo $cool_feature->get_memory_limit() . PHP_EOL; # 40M
echo $better_feature->get_memory_limit() . PHP_EOL; # 40M

$cool_feature->set_memory_limit( '80M' );

echo $better_feature->get_memory_limit() . PHP_EOL ; # what would you expect and why?

static-dynamic.php

<?php

/**
 * shows the difference between static and dynamic class properties
 *
 */

namespace Static_Or_Dynamic;

/**
 * a class with a static and a dynamic property
 *
 * we want to represent devices in a single network domain
 */

class Device {

	/**
	 * network domain
	 *
	 * @var string
	 */
	protected static $domain = 'home_network';

	/**
	 * the type of the device
	 *
	 * e.g. pc, tablett, smartphone
	 *
	 * @var string
	 */
	protected $type = '';

	/**
	 * the device name
	 *
	 * @var string
	 */
	protected $name = '';

	/**
	 * create a new device
	 *
	 * @param string $type
	 */
	public function __construct( $type, $name ) {

		$this->type = $type;
		$this->name = $name;
	}

	/**
	 * dynamic setter/getter
	 */
	public function set_name( $name ) {

		$this->name = $name;
	}

	public function get_name() {

		return $this->name;
	}

	public function get_type() {

		return $this->type;
	}

	/**
	 * static setter and getter
	 */
	public static function set_domain( $domain ) {

		self::$domain = $domain;
	}

	public static function get_domain() {

		return self::$domain;
	}
}

$pc         = new Device( 'PC', 'My Desktop Computer' );
$smartphone = new Device( 'Smartphone', 'My Android Phone' );

echo $pc->get_name(); # My Desktop Computer
echo $smartphone->get_name(); # My Android Phone

# Change the name of the PC
$pc->set_name( "Jane's Desktop Computer" );

echo $smartphone->get_name(); # My Android Phone

# now change the network domain
$pc::set_domain( 'office_network' );

echo $smartphone::get_domain(); # what do you expect?