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https://www.virendrachandak.com/techtalk/php-isset-vs-empty-vs-is_null/
http://www.zomeoff.com/php-fast-way-to-determine-a-key-elements-existance-in-an-array/

Checking a value directly

$people = array(
    'one'   => 'Eric',  // true
    'two'   => '',      // false
    'three' => null,    // false
    'four'  => 0,       // false
    'five'  => '0',     // false
    'six'   => ' ',     // true
);

if ( $people['one'] ) {};

isset

• This will check to see if there is a value for an item in an array.
• This will return true if the value is set to ''.
• This will return false if the value is set to null.

$people = array(
    'one'   => 'Eric',  // true
    'two'   => '',      // true
    'three' => null,    // false
    'four'  => 0,       // true
    'five'  => '0',     // true
    'six'   => ' ',     // true
);

if ( isset( $people['one'] ) ) {};

Only returns false if the value is set to null.

empty

• Will return true if set to '', or null
• These are values empty will evaluate as empty.
  • "" (an empty string)
  • 0 (0 as an integer)
  • 0.0 (0 as a float)
  • "0" (0 as a string)
  • null
  • false
  • array() (an empty array)
  • var $var; (a variable declared, but without a value in a class)

$people = array(
    'one'   => 'Eric', // false
    'two'   => '',     // true
    'three' => null,   // true
    'four'  => 0,      // true
    'five'  => '0',    // true
    'six'   => ' ',    // false
);

if ( empty( $people['one'] ) ) {};

Takeaways

So the big difference here is that empty will also return true if the value is ''. This makes it the more safe function to use if you want to make sure '' isn't accepted.