Data Types   integer, float, boolean, string, array, object, resource, NULL     Variable Declarations   $variablename = <value>;   $anothervariable =& $variablename; (Assign by Reference)     Declare Array   $arrayname = array();     Initialize Array   $arrayname = array(<value1>, <value2>, <value3>);   $arrayname = array(<key> => <value>, <key> => <value>); (Define Keys)   $multiarray = array(<key> => array(<value1>,<value2>)); (Multi-dimensional)     Common Array Functions   sort(<array>); (Sort array assigns new keys)   asort(<array>); (Sort array maintain keys)   rsort(<array>); (Sort array in reverse, new keys)   arsort(<array>); (Sort array in reverse, maintain keys)   count(<array>); (Count elements)   count(<array>,COUNT_RECURSIVE); (Count multidimensional array)   array_push(<array>,<value>); (Push item onto end of array)   array_pop(<array>...