Description:
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Now that property_exists returns true for all private properties, there is no way (I know) to test that a property is private before writing to it and causing a fatal error. 

The previous behaviour was wrong - private properties that are in scope should return true - but the new code is worse. 
See Bug #50810: property_exists does not work for private 

I think property_exists should return true for writable properties...
  which implies a scope... 
    if the user coded $this then the scope is the current scope. 

If the use gives a class name, then the scope is that class. 

I have not managed to re-code the routine below to work in 5.3.0 (let alone both 5.3.? and 5.2.?). 



Test script:
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public function setField($fld, $value) {
      // set the fld to the value
      if (property_exists($this,$fld)) {
         $this->$fld = $value;  // handles public and protected fields 
         return;
      }
      $this->setMapped($fld,$value); // for private vars.
   }

Create a class with the above function. Create a sub-class with a private variable. Call it on the sub-class with fldname and value. 
In 5.3.0 this crashes with a fatal error. 
In 5.2.9 it calls setMapped.