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<?php
/********* DO NOT COPY THE PARTS ABOVE THIS LINE *********/

/*
 * Change meta robots using Yoast SEO
 * Credit: Yoast development team
 * Last Tested: Jun 19 2017 using Yoast SEO 4.9 on WordPress 4.8
 */

add_filter( 'wpseo_robots', 'yoast_seo_robots_remove_search' );

function yoast_seo_robots_remove_search( $robots ) {
  if ( is_search() ) {
    return false;
  } else {
    return $robots;
  }
}

<?php
/********* DO NOT COPY THE PARTS ABOVE THIS LINE *********/

/*
 * Remove meta robots from Yoast SEO
 * Credit: Yoast development team
 * Last Tested: Mar 01 2017 using Yoast SEO 4.4 on WordPress 4.7.2
 */

add_filter( 'wpseo_robots', '__return_false' );

<?php
/********* DO NOT COPY THE PARTS ABOVE THIS LINE *********/

/*
 * Replace Disallow with Allow Generated Robots.txt
 * Credit: Unknown
 * Last Tested: Unknown
 */

add_filter('robots_txt','custom_robots');

function custom_robots($output) {
     $public = get_option( 'blog_public' );
     if ( '0' != $public )
     return str_replace('Disallow','Allow',$output);
}