简单谈谈PHP中的include、include_once、require以及require_once语句，requireonceinclude
简单谈谈PHP中的include、include_once、require以及require_once语句，requireonceinclude
1.include语句
使用include语句可以告诉PHP提取特定的文件，并载入它的全部内容
<?php inlude "fileinfo.php"; //此处添加其他代码 ?>
2.include_once语句
每次使用include语句时，它都会重新将请求的文件导入，即使这个文件已经被导入过。例如，假定fileinfo.php文件包含许多函数，我们使用include语句将他导入到现有的文件中，然后我们又导入了一个包含fileinfo.php的文件，通过嵌套，我们已经将fileinfo.php文件导入了两次，这就会产生错误，因为我们试图多次定义同名的变量或函数。为了避免这样的事情发生，我们使用include_once语句来代替include语句
<?php include_once "fileinfo.php"; //此处添加其他代码 ?>
此时，如果在相同的文件中遇到另一个include或include_once语句时，PHP会检查它是否已经被导入过，如果是，就忽略它。
3.require和require_once语句
使用include和include_once语句的潜在问题是：PHP只会试图导入被请求导入的文件，即使该文件没有被找到，程序依旧会执行。
当我们绝对需要导入一个文件时，使用require语句，对于使用require_once语句的原因也是一样的，在这就不再赘述了。
<?php require_once "fileinfo.php"; //此处添加其他代码 ?>
总的来说，我们应该坚持使用require_once语句。
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