include is a special language construct and not a function, it doesn’t need paranthesis for the paremeter list:
include() is a special language construct, parentheses are not needed around its argument. Take care when comparing return value.
In fact it has just one parameter and wrapping it in additional parenthesis doesn’t change anything:
1 ≡ (1) ≡ ((1)) ≡ (((1))) ≡ …
So your statement is identical to this (the paremter is just wrapped):
require_once (('abc.php') or die("oops"));
So we have a boolean expression as parameter that is either true or false. And that values have the string equivalent of
var_dump((string) true === "1");
var_dump((string) false === "");
That’s the reason why get this Failed opening required '1' error message.
But using parenthesis on the right place like this makes it work like you want it:
(@include_once 'abc.php') or die("oops");
'abc.php' is clearly the parameter and the disjunction with
die("oops") is performed on the return value of
@ operator is just to ignor the error message
includ_once will throw if the file does not exist.
print is also a special language construct and works the same way.