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<?php
//in form
$my_checkbox = isset( $instance['my_checkbox'] ) ? true : false;

?>
<p>
			<label for="<?php echo $this->get_field_id( 'my_checkbox' ); ?>"><?php _e( 'Checkbox Label' ); ?></label>
			<input class="checkbox" type="checkbox" id="<?php echo $this->get_field_id( 'my_checkbox' ); ?>" name="<?php echo $this->get_field_name( 'my_checkbox' ); ?>" <?php checked( 1, $my_checkbox, 1 ); ?>>
</p>

<?php
//in update
$instance[ 'my_checkbox' ] =  $new_instance[ 'my_checkbox' ];

//in widget
$my_checkbox = isset( $instance[ 'my_checkbox' ] ) ? true : false ;