Php Singleton object format

Php Singleton object mode? Suppose we need to write a class to operate the database and meet the following requirements: ① the SqlHelper class can only have one instance (not many) ② SqlHelper class must be able to create this instance on its own ③ This instance must be provided to the entire system on its own. In other words, multiple objects share a memory area, for example, object A sets the object mode of A php Singleton.
? Assume that we need to write a class to operate the database and meet the following requirements:

① The SqlHelper class can have only one instance (no more)
② The SqlHelper class must be able to create this instance by itself
③ You must provide this instance to the entire system. In other words, multiple objects share A memory area. for example, if object A has set certain attribute values, object B, C can also access these attribute values (this problem is well illustrated in the example at the end)


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