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);
?>

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.

<?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;

?>

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"
}

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 !  :)

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.

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.