How to get instance of a specific class in PHP?

[2shardDt5]

I need to check if there exists an instance of \[code\]class_A\[/code\] ,and if there does exist, get that instance.How to do it in PHP?As always,I think an simple example is best.Now my problem has become:\[code\]$ins = new class_A();\[/code\]How to storing the instance in a static member variable of \[code\]class_A\[/code\] when instantiating?It'll be better if the instance can be stored when calling \[code\]__construct()\[/code\].Say,it should work without limitation on how it's instantiated.