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Created attachment 4656 [details]
Patch to correct issue as described

The code for the function getExprParser returns a handle to a new instance of ExprParser on every call, rather than a pre-existing handle if one is already set.

Here is the current code:

    function &getExprParser() {
        if ( !isset( $this->mExpr ) ) {
            if ( !class_exists( 'ExprParser' ) ) {
                require( dirname( __FILE__ ) . '/Expr.php' );
            }
            $this->mExprParser = new ExprParser;
        }
        return $this->mExprParser;
    }

The variable checked in the if statement, mExpr, is not defined as a class variable in the ExtParserFunctions class.  I believe this to be a typo / refactoring artifact, and that it should instead check to see if mExprParser is set.

    function &getExprParser() {
        if ( !isset( $this->mExprParser ) ) {
            if ( !class_exists( 'ExprParser' ) ) {
                require( dirname( __FILE__ ) . '/Expr.php' );
            }
            $this->mExprParser = new ExprParser;
        }
        return $this->mExprParser;
    }

I've attached a patch with the correction as well.