Class: Converts plain text links into <a> HTML elements (supports `rel` and `target`)

convert_links.php

<?php
/**
 * Converts URLs in a string into hyperlinks.
 *
 * Example: <?php echo new ConvertLinks( 'Text containing a link http://gist.github.com', 'external', '_blank' ); ?>
 * Output: Text containing a link <a href="http://gist.github.com" rel="external" target="_blank">http://gist.github.com</a>
 *
 * @param string $input  Text containing links
 * @param string $rel    Rel attribute value (optional)
 * @param string $target Target attribute value (optional)
 *
 * @see https://developer.wordpress.org/reference/functions/make_clickable/
 *
 * @return string
 */
class ConvertLinks {

	protected static $input;
	protected static $rel;
	protected static $target;
	protected $output;

	function __construct( $input = '', $rel = '', $target = '' ) {

		$this::$input  = $input;
		$this::$rel    = $rel;
		$this::$target = $target;

		$pattern = '#([\s>])([\w]+?://[\w\\x80-\\xff\#$%&~/.\-;:=,?@\[\]+]*)#is';
		preg_match_all( $pattern, $this::$input, $urls );

		// Bail out if we have nothing to work with
		if ( empty( $urls ) || false === isset( $urls[0] ) ) return $this::$input;

		// Loop through each link
		foreach ( $urls[0] as $url ) {

			$output = preg_replace_callback( $pattern, function( $matches ) {

				$attributes = array();
				$prefix     = $matches[1];
				$suffix     = '';
				$url        = $matches[2];

				if ( empty( $url ) ) return $matches[0];

				// Move special characters outside of link
				// Covers links that end or split sentences
				while ( true === in_array( substr( $url, -1 ), array( '.', ',', ';', ':' ) ) ) {
					$suffix .= substr( $url, -1 );
					$url = substr( $url, 0, strlen( $url ) -1 );
				}

				// Add rel="" attribute
				if ( ! empty( $this::$rel ) ) {
					$rel = sprintf( 'rel="%s"', htmlspecialchars( $this::$rel, ENT_QUOTES ) );
					array_push( $attributes, $rel );
				}

				// Add target="" attribute
				if ( ! empty( $this::$target ) ) {
					$target = sprintf( 'target="%s"', htmlspecialchars( $this::$target, ENT_QUOTES ) );
					array_push( $attributes, $target );
				}

				// Compile attributes
				$safe_attributes = empty( $attributes ) ? '' : ' ' . join( ' ', $attributes );

				// Sanitize URL
				$safe_url = htmlspecialchars( $url, ENT_QUOTES );

				// Build HTML tag
				$link = sprintf( '<a href="%s"%s>%s</a>', $safe_url, $safe_attributes, $safe_url );

				return $prefix . $link . $suffix;

			// Add extra space in case the string starts with a link
			}, ' ' . $this::$input );

			// Remove nested links
			$output = preg_replace( '#(<a([ \r\n\t]+[^>]+?>|>))<a [^>]+?>([^>]+?)</a></a>#i', '$1$3</a>', $output );

			// Rremove extra whitespace and update output
			$this->output = trim( $output );
		}
	}

	// Return output as a string
	function __toString( ) {
		return $this->output;
	}
}

// Give it a spin!
echo new ConvertLinks( 'http://gist.github.com So http://gist.github.com?param=value many http://gist.github.com#supports/hasbangs/ links http://gist.github.com?param=combo#works/as/well!', 'external', '_blank' );