WP_Widget::get_field_name( string $field_name )

Source

File: wp-includes/class-wp-widget.php

public function get_field_name( $field_name ) {
		$pos = strpos( $field_name, '[' );

		if ( false !== $pos ) {
			// Replace the first occurrence of '[' with ']['.
			$field_name = '[' . substr_replace( $field_name, '][', $pos, strlen( '[' ) );
		} else {
			$field_name = '[' . $field_name . ']';
		}

		return 'widget-' . $this->id_base . '[' . $this->number . ']' . $field_name;
	}