目的是！$和差异b / w isset和空？ [重复]

This question already has an answer here:

  1. What is the purpose of !$ in PHP?
  2. What is the difference between isset and empty?
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  1. !$ means NOT thus if (!$a) and $a == TRUE then it is NOT TRUE
    Reference: http://www.php.net/manual/en/language.operators.logical.php

  2. ISSET returns TRUE if the variable exists and has a value other than NULL
    EMPTY checks to see if a variable is empty.
    Reference: http://php.net/manual/en/function.isset.php http://php.net/manual/en/function.empty.php