test_isset.php

<?php
/**
 * different from empty is_null isset
 */

$t = '';
//is a string "" !isset($t) return true ;!is_null($t) return ture;empty ($t) return true;
$t; 
// = "Hello"; //is NULL;!isset($t) return false ;!is_null($t) return false empty ($t) return ture;
//first $t is init ;second $t have the same address of memory but doesn't init
//not overwrite first $t's value
//NULL and empty string all empty to empty function
//if(isset($t))
if (!is_null($t)){
    var_dump($t);
}
else
    echo '$t存在但为NULL';
?>

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