Note that you can't use this to convert a string 'true' or 'false' to a boolean variable true or false as a string 'false' is a boolean true. The empty string would be false instead...

<?php
$var = "true";
settype($var, 'bool');
var_dump($var); // true

$var = "false";
settype($var, 'bool');
var_dump($var); // true as well!

$var = "";
settype($var, 'bool');
var_dump($var); // false
?>

Just a quick note, as this caught me out very briefly:

settype() returns bool, not the typecasted variable - so:

$blah = settype($blah, "int"); // is wrong, changes $blah to 0 or 1
settype($blah, "int"); // is correct

Hope this helps someone else who makes a mistake.. ;)

Using settype is not the best way to convert a string into an integer, since it will strip the string wherever the first non-numeric character begins.  The function intval($string) does the same thing.

If you're looking for a security check, or to strip non-numeric characters (such as cleaning up phone numbers or ZIP codes),  try this instead:

<?
     $number=ereg_replace("[^0-9]","",$number);
?>

<?php
/*
This example works 4x faster than settype() function in PHP-CGI 5.4.13 and
8x faster in PHP-CGI 7.1.3(x64) for windows
*/

$v = '12345';

$v = (int)$v;
$v = (string)$v;

?>

you must note that this function will not set the type permanently! the next time you set the value of that variable php will change its type as well.

If you attempt to convert the special $this variable from an instance method (only in classes) :
* PHP will silently return TRUE and leave $this unchanged if the type was 'bool', 'array', 'object' or 'NULL'
* PHP will generate an E_NOTICE if the type was 'int', 'float' or 'double', and $this will not be casted
* PHP will throw a catchable fatal error when the type is 'string' and the class does not define the __toString() method
Unless the new variable type passed as the second argument is invalid, settype() will return TRUE. In all cases the object will remain unchanged.
<?php
    // This was tested with PHP 7.2
    class Foo {
        function test() {
            printf("%-20s %-20s %s\n", 'Type', 'Succeed?', 'Converted');
            
            // settype() should throw a fatal error, as $this cannot be re-assigned
            printf("%-20s %-20s %s\n", 'bool', settype($this, 'bool'), print_r($this, TRUE));
            printf("%-20s %-20s %s\n", 'int', settype($this, 'int'), print_r($this, TRUE));
            printf("%-20s %-20s %s\n", 'float', settype($this, 'float'), print_r($this));
            printf("%-20s %-20s %s\n", 'array', settype($this, 'array'), print_r($this, TRUE));
            printf("%-20s %-20s %s\n", 'object', settype($this, 'object'), print_r($this, TRUE));
            printf("%-20s %-20s %s\n", 'unknowntype', settype($this, 'unknowntype'), print_r($this, TRUE));
            printf("%-20s %-20s %s\n", 'NULL', settype($this, 'NULL'), print_r($this, TRUE));
            printf("%-20s %-20s %s\n", 'string', settype($this, 'string'), print_r($this, TRUE));
        }
    }
    $a = new Foo();
    $a->test();
?>
Here is the result :
Type                 Succeed?             Converted
bool                 1                    Foo Object
(
)

Notice: Object of class Foo could not be converted to int in C:\php\examples\oop-settype-this.php on line 9

int                  1                    Foo Object
(
)

Notice: Object of class Foo could not be converted to float in C:\php\examples\oop-settype-this.php on line 10

float                1                    Foo Object
(
)

array                1                    Foo Object
(
)

object               1                    Foo Object
(
)

Warning: settype(): Invalid type in C:\php\examples\oop-settype-this.php on line 14

unknowntype                               Foo Object
(
)

NULL                 1                    Foo Object
(
)

Catchable fatal error: Object of class Foo could not be converted to string in C:\php\examples\oop-settype-this.php on line 15

If the class Foo implements __toString() :
<?php
    class Foo {
        // ...
        function __toString() {
            return 'Foo object is awesome!';
        }
        // ...
    }
?>
So the first code snippet will not generate an E_RECOVERABLE_ERROR, but instead print the same string as for the other types, and not look at the one returned by the __toString() method.

Hope this helps !  :)

Note that "boolean", "integer" and "double" are not canonical type names.
Instead of them I use canonical names : "bool",  "int" and "float".

Please note: 

When using settype to convert indexed arrays to objects, the properties of the typed object will be integers:

A brief example:

$a = ['1', '2'];

settype($a, 'object');

var_dump($a);

// output
object(stdClass)#1 (2) {
  ["0"]=>
  string(1) "1"
  ["1"]=>
  string(1) "2"
}

using (int) insted of the settype function works out much better for me. I have always used it. I personally don't see where settype would ever come in handy.

This settype() behaviour seems consistent to me. Quoting two sections from the manual:

"When casting from a scalar or a string variable to an array, the variable will become the first element of the array: "
<pre>
2 $var = 'ciao';
3 $arr = (array) $var;
4 echo $arr[0];  // outputs 'ciao' 
</pre>

And if (like your code above) you do a settype on an empty variable, you'll end up with a one element array with an empty (not unset!) first element. So appeanding to it will start appending at index 1. As for why reset() doesn't do anything:

"When you assign a value to an array variable using empty brackets, the value will be added onto the end of the array."

It doesn't matter where the array counter is; values are added at the end, not at the counter.