Casting (string) „true“ to (boolean) true/ string „false“ to (boolean) false

~$ php -r "var_dump( filter_var( "false", FILTER_VALIDATE_BOOLEAN ) ); "
bool(false)
~$ php -r "var_dump( filter_var( "true", FILTER_VALIDATE_BOOLEAN ) ); "
bool(true)
~$ php -r "var_dump( filter_var( 0, FILTER_VALIDATE_BOOLEAN ) ); "
bool(false)
~$ php -r "var_dump( filter_var( 1, FILTER_VALIDATE_BOOLEAN ) ); "
bool(true)