include，include_once，require，require_once的区别，requireonceinclude
include，include_once，require，require_once的区别，requireonceinclude
1、include,require在其被调用的位置处包含一个文件。
2、include_once,require_once函数的作用与include相同，不过它会首先验证是否已包含该文件。如果已经包含，则不再执行include_once。其他同include一样。
3、require与include最主要的区别，a、require出错时，脚本将停止运行，而include出错的情况下，脚本将继续执行。b、无论require的位置如何，制定文件都将包含到出现require的脚本中。例如，即使require放在计算结果为假的if语句中，依然会包含指定文件。
4、使用require_once可以解决文件被覆盖的问题。 require_once函数确保文件只包含一次。在遇到require_once后，后面再试图包含相同的文件时将被忽略。
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