Add a protocol to URLs from WordPress post content

add_protocol_in_post_content.php

<?php

/**
 * Fix the URLs from the content.
 *
 * This function will add protocol to URLs which has no protocol added.
 *
 * For example,
 *
 * google.com         --> http://google.com
 * https://google.com --> https://google.com
 * http://google.com  --> http://google.com
 *
 * @param  string $content Content.
 * @return string          Content with fixed URLs.
 */
function fix_content_urls( $content = '' ) {

	// Bail, if anything goes wrong.
	if ( empty( $content ) ) {
		return;
	}

	// Get all the links.
	preg_match_all( '/<a[^>]+href=([\'"])(?<href>.+?)\1[^>]*>/i', $content, $result );

	$search = $replace = array();

	// Create search & replace array.
	if ( isset( $result['href'] ) && ! empty( $result['href'] ) ) {

		foreach ( $result['href'] as $url ) {

			$parsed_url = parse_url( $url );

			// If no protocol found, then add it for replacement.
			if ( ! empty( $parsed_url ) && ! isset( $parsed_url['scheme'] ) ) {
				$search[]  = $url; // Original URL.
				$replace[] = esc_url( $url ); // Fixed URL.
			}
		}

		// Fix the URLs.
		$content = str_replace( $search, $replace, $content );

		// Freeup the variables.
		unset( $search );
		unset( $replace );

	}

	return $content;
}