How to convert string to boolean php

Strings always evaluate to boolean true unless they have a value that's considered "empty" by PHP (taken from the documentation for empty):

1. "" (an empty string);
2. "0" (0 as a string)

If you need to set a boolean based on the text value of a string, then you'll need to check for the presence or otherwise of that value.

$test_mode_mail = $string === 'true'? true: false;

EDIT: the above code is intended for clarity of understanding. In actual use the following code may be more appropriate:

$test_mode_mail = ($string === 'true');

or maybe use of the filter_var function may cover more boolean values:

filter_var($string, FILTER_VALIDATE_BOOLEAN);

filter_var covers a whole range of values, including the truthy values "true", "1", "yes" and "on". See here for more details.

This method was posted by @lauthiamkok in the comments. I'm posting it here as an answer to call more attention to it.

Depending on your needs, you should consider using filter_var() with the FILTER_VALIDATE_BOOLEAN flag.

filter_var(    true, FILTER_VALIDATE_BOOLEAN); // true
filter_var(    'true', FILTER_VALIDATE_BOOLEAN); // true
filter_var(         1, FILTER_VALIDATE_BOOLEAN); // true
filter_var(       '1', FILTER_VALIDATE_BOOLEAN); // true
filter_var(      'on', FILTER_VALIDATE_BOOLEAN); // true
filter_var(     'yes', FILTER_VALIDATE_BOOLEAN); // true

filter_var(   false, FILTER_VALIDATE_BOOLEAN); // false
filter_var(   'false', FILTER_VALIDATE_BOOLEAN); // false
filter_var(         0, FILTER_VALIDATE_BOOLEAN); // false
filter_var(       '0', FILTER_VALIDATE_BOOLEAN); // false
filter_var(     'off', FILTER_VALIDATE_BOOLEAN); // false
filter_var(      'no', FILTER_VALIDATE_BOOLEAN); // false
filter_var('asdfasdf', FILTER_VALIDATE_BOOLEAN); // false
filter_var(        '', FILTER_VALIDATE_BOOLEAN); // false
filter_var(      null, FILTER_VALIDATE_BOOLEAN); // false

The String "false" is actually considered a "TRUE" value by PHP. The documentation says:

To explicitly convert a value to boolean, use the (bool) or (boolean) casts. However, in most cases the cast is unnecessary, since a value will be automatically converted if an operator, function or control structure requires a boolean argument.

See also Type Juggling.

When converting to boolean, the following values are considered FALSE:

• the boolean FALSE itself

• the integer 0 (zero)

• the float 0.0 (zero)

• the empty string, and the string "0"

• an array with zero elements

• an object with zero member variables (PHP 4 only)

• the special type NULL (including unset variables)

• SimpleXML objects created from empty tags

Every other value is considered TRUE (including any resource).

so if you do:

$bool = (boolean)"False";

or

$test = "false";
$bool = settype($test, 'boolean');

in both cases $bool will be TRUE. So you have to do it manually, like GordonM suggests.