Parsing a string into a boolean value in PHP

There is a native PHP method of doing this which uses PHP's filter_var method:

$bool = filter_var($value, FILTER_VALIDATE_BOOLEAN);

According to PHP's manual:

Returns TRUE for "1", "true", "on" and "yes". Returns FALSE otherwise.

If FILTER_NULL_ON_FAILURE is set, FALSE is returned only for "0", "false", "off", "no", and "", and NULL is returned for all non-boolean values.

In PHP only "0" or the empty string coerce to false; every other non-empty string coerces to true. From the manual:

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

• the empty string, and the string "0"

You need to write your own function to handle the strings "true" vs "false". Here, I assume everything else defaults to false:

function isBoolean($value) {
   if ($value === "true") {
      return true;
   } else {
      return false;
   }
}

On a side note that could easily be condensed to

function isBoolean($value) {
   return $value === "true";
}

The reason is that all strings evaluate to true when converting them to boolean, except "0" and "" (empty string).

The following function will do exactly what you want: it behaves exactly like PHP, but will also evaluates the string "false" as false:

function isBoolean($value) {
   if ($value && strtolower($value) !== "false") {
      return true;
   } else {
      return false;
   }
}

The documentation explains that: http://php.net/manual/en/language.types.boolean.php :

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
• the special type NULL (including unset variables)
• SimpleXML objects created from empty tags

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