C VS PHP

C.c

uint x[3] = {0x42, 0x69, 0x13};
int i = 0;
    
printf("Res : %x\n", x[i++] << 16 | x[i++] << 8 | x[i++]);
/* Output 0x424242 */

Diff.c

int i = 1;
printf("Res : %d\n", i++ + i++);


Result with llvm clang and gcc :

para@Bohr:~/dev/libmpqsc2/$ clang x.c
para@Bohr:~/dev/libmpqsc2/$ ./a.out 
Res : 3
para@Bohr:~/dev/libmpqsc2/$ gcc x.c
para@Bohr:~/dev/libmpqsc2/$ ./a.out 
Res : 2

php.php

<?php
$x = array(0x42, 0x69, 0x13);
$i = 0;
	
printf("Res : %x\n", $x[$i++] << 16 | $x[$i++] << 8 | $x[$i++]);
/* Output 0x426913 */
?>

Different "++" execution order

Different | versus << / >> operators precedence maybe ?

http://c-faq.com/expr/seqpoints.html

Since there's no good way to define it, the Standard declares that it is undefined, and that portable programs simply must not use such constructs.