isset()检查变量是否定义，而empty()检查定义的变量是否是空值:
$g = array(); echo isset($g)?"isset(g) is true \n":"isset(g) is false \n"; echo empty($g)?"empty(g) is true \n":"empty(g) is false \n";
运行结果：
isset(g) is true //$g的值为非null empty(g) is true //$g虽然已定义，但值为空
isset()检查变量是否定义，而empty()检查定义的变量是否是空值:
$g = array(); echo isset($g)?"isset(g) is true \n":"isset(g) is false \n"; echo empty($g)?"empty(g) is true \n":"empty(g) is false \n";
运行结果：
isset(g) is true //$g的值为非null empty(g) is true //$g虽然已定义，但值为空
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