Swapping two variables without using a third variable is an interesting problem that can be solved using various techniques. Here are a few different methods to achieve variable swapping in PHP:

**1. Using Arithmetic:**

```
$a = 5;
$b = 10;
$a = $a + $b; // $a now holds the sum
$b = $a - $b; // $b now holds the original value of $a
$a = $a - $b; // $a now holds the original value of $b
```

**2. Using Bitwise XOR:**

```
$a = 5;
$b = 10;
$a = $a ^ $b;
$b = $a ^ $b;
$a = $a ^ $b;
```

**3. Using Addition and Subtraction:**

```
$a = 5;
$b = 10;
$a = $a + $b;
$b = $a - $b;
$a = $a - $b;
```

**4. Using Multiplication and Division:**

```
$a = 5;
$b = 10;
$a = $a * $b;
$b = $a / $b;
$a = $a / $b;
```

**5. Using List and Explode:**

```
$a = 5;
$b = 10;
list($b, $a) = explode(',', "$a,$b");
```

**6. Using XOR and Addition:**

```
$a = 5;
$b = 10;
$a = $a ^ $b;
$b = $a ^ $b;
$a = $a ^ $b;
$a = $a + $b;
$b = $a - $b;
$a = $a - $b;
```

**7. Using List and Array:**

```
$a = 5;
$b = 10;
list($a, $b) = [$b, $a];
```

**8. Using Destructuring Assignment (PHP 7.1+):**

```
$a = 5;
$b = 10;
[$a, $b] = [$b, $a];
```

**9. Using XOR, Subtraction, and Negation:**

```
$a = 5;
$b = 10;
$a = $a ^ $b;
$b = $a ^ $b;
$a = $a ^ $b;
$b = -$b;
```

Remember that while these techniques are interesting, in real-world scenarios, it’s often clearer and more maintainable to use a temporary variable for variable swapping.