Do you have Zend OPcache enabled? Because with it, it is smart, and removes the whole if part of it. See what VLD says with OPCache on:

line     #* E I O op                           fetch          ext  return  operands

3 0 E > EXT_STMT
6 1 EXT_STMT
2 ECHO 1
9 3 > RETURN 1

It doesn't generate anything on line 5 (where the elseif is at).

Even without OPcache, you're going to be out of luck, as PHP doesn't generate an EXT_STMT for "elseif" (only a JMPZ on line 5):

line     #* E I O op                           fetch          ext  return  operands

3 0 E > EXT_STMT
1 > JMPZ <false>, ->3
2 > > JMP ->6
5 3 > > JMPZ <true>, ->6
6 4 > EXT_STMT
5 ECHO 1
9 6 > > RETURN 1

although it does for "else if":

line     #* E I O op                           fetch          ext  return  operands

3 0 E > EXT_STMT
1 > JMPZ <false>, ->3
2 > > JMP ->7
5 3 > EXT_STMT
4 > JMPZ <true>, ->7
6 5 > EXT_STMT
6 ECHO 1
9 7 > > RETURN 1

I'm afraid there is nothing I can do here, and I would recommend you file a bug with the PHP team at https://bugs.php.net in order for them to fix that.