$undefined;
$null = null;
$empty = '';
$space = ' ';
$value = 'value';
$empty_array = array();
$array = array();
$array[0] = '';

[isset]

isset($undefined):

isset($null):

isset($empty): 1

isset($space): 1

isset($value): 1

isset($empty_array): 1

isset($array): 1

[is true]

$undefined == true:

$null == true:

$empty == true:

$space == true: 1

$value == true: 1

$empty_array == true:

$array == true: 1

[is null]

$undefined == null: 1

explode

explode(',', 'ab'): Array

explode(',', 'ab, cd'): Array

a/b