Source

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

public function print_inline_style( $handle, $echo = true ) {
		$output = $this->get_data( $handle, 'after' );

		if ( empty( $output ) ) {
			return false;
		}

		$output = implode( "\n", $output );

		if ( ! $echo ) {
			return $output;
		}

		printf(
			"<style id='%s-inline-css'%s>\n%s\n</style>\n",
			esc_attr( $handle ),
			$this->type_attr,
			$output
		);

		return true;
	}