Not unique to PHP in the slightest. In C sizeof(int, 2) will return a parse error, because sizeof is an unary operator despite looking like a function (t

Not unique to PHP in the slightest. In C sizeof(int, 2) will return a parse error, because sizeof is an unary operator despite looking like a function (the parentheses are mandatory when using it with a type name). In PHP, isset is implemented as a "language construct", essentially like another operator.