Description:
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If I pass the optional third parameter in DEFINE method as TRUE, it is allowing me to override the existing value of a constant. This is not the expected behavior as constant's value should not be changed once it is declared.  

Test script:
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define("GREETING", "Hello you.",true);
echo GREETING;  // outputs "Hello you." 
echo "before changing the constant value <br/>"; //before changing the constant value

define("GREETING", "sample value.");
echo GREETING; // outputs "sample value."
echo "<br/>";


Expected result:
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constant's value should not be changed once it is declared.

Actual result:
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it is allowing me to override the existing value of a constant.