## Why is this an issue?

When the same code is duplicated in two or more separate branches of a conditional, it can make the code harder to understand, maintain, and can potentially introduce bugs if one instance of the code is changed but others are not.

Having two `cases` in a `switch` statement or two branches in an `if` chain with the same implementation is at best duplicate code, and at worst a coding error.

    if ($a >= 0 && $a < 10) {
      doFirst();
      doTheThing();
    }
    else if ($a >= 10 && $a < 20) {
      doTheOtherThing();
    }
    else if ($a >= 20 && $a < 50) {
      doFirst();
      doTheThing();  // Noncompliant; duplicates first condition
    }

    switch ($i) {
      case 1:
        doFirst();
        doSomething();
        break;
      case 2:
        doSomethingDifferent();
        break;
      case 3:  // Noncompliant; duplicates case 1's implementation
        doFirst();
        doSomething();
        break;
      default:
        doTheRest();
    }

If the same logic is truly needed for both instances, then:

- in an `if` chain they should be combined

    if (($a >= 0 && $a < 10) || ($a >= 20 && $a < 50)) {
      doFirst();
      doTheThing();
    }
    else if ($a >= 10 && $a < 20) {
      doTheOtherThing();
    }

- for a `switch`, one should fall through to the other

    switch ($i) {
      case 1:
      case 3:
        doFirst();
        doSomething();
        break;
      case 2:
        doSomethingDifferent();
        break;
      default:
        doTheRest();
    }

### Exceptions

Blocks in an `if` chain that contain a single line of code are ignored, as are blocks in a `switch` statement that contain a single line of code with or without a following `break`.

    if ($a >= 0 && $a < 10) {
      doTheThing();
    }
    else if ($a >= 10 && $a < 20) {
      doTheOtherThing();
    }
    else if ($a >= 20 && $a < 50) {
      doTheThing();  // no issue, usually this is done on purpose to increase the readability
    }

But this exception does not apply to `if` chains without `else`-s, or to `switch`-es without default clauses when all branches have the same single line of code. In the case of `if` chains with `else`-s, or of `switch`-es with default clauses, rule {rule:php:S3923} raises a bug.

    if ($a >= 0 && $a < 10) {
      doTheThing();
    }
    else if ($a >= 20 && $a < 50) {
      doTheThing();   //Noncompliant; this might have been done on purpose but probably not
    }

## Resources

### Related rules

- {rule:php:S3923} - All branches in a conditional structure should not have exactly the same implementation