language_construct_and_function

1.Calling isset happens to be faster then strlen because unlike strlen , isset is a language construct and not a function meaning that it's execution does not require function lookups and lowercase.
调用isset恰巧比strlen快，因为与后者不同的是，isset作为一种语言结构，意味着它的执行不需要函数查找和字母小写化。
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