Issues getting PHP to display in category pages

Based on your category.php I assume you’re using the Zeal theme, which means that get_template_part( 'template-parts/content', 'category' ) probably spits out something like this, https://themes.trac.wordpress.org/browser/zeal/1.0.8/template-parts/content-category.php, but do correct me, if I’m wrong.

Based on this assumption I’d say there is no the_content hook available in your loop, so your crunchify_social_sharing_buttons function never gets executed. But you should be able to use the_excerpt filter, as the_excerpt is used in the template part file, to achieve the same result. To do that you need to slightly modify your code – just move the social media link generation to a separate function, which you then hook to both the_content and the_excerpt.

Like so,

add_filter( 'the_content', 'the_content_crunchify_social_sharing_buttons');
function the_content_crunchify_social_sharing_buttons($content) {
  return (is_singular() || is_home()) ? crunchify_social_sharing_buttons($content) : $content;
}

add_filter( 'the_excerpt', 'the_excerpt_crunchify_social_sharing_buttons');
function the_excerpt_crunchify_social_sharing_buttons($excerpt) {
  return (is_archive()) ? crunchify_social_sharing_buttons($excerpt) : $excerpt;
}

function crunchify_social_sharing_buttons($content) {
  global $post;

  // Get current page URL 
  $crunchifyURL = urlencode(get_permalink());

  // Get current page title
  $crunchifyTitle = htmlspecialchars(urlencode(html_entity_decode(get_the_title(), ENT_COMPAT, 'UTF-8')), ENT_COMPAT, 'UTF-8');
  // $crunchifyTitle = str_replace( ' ', '%20', get_the_title());

  // Get Post Thumbnail for pinterest
  $crunchifyThumbnail = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'full' );

  // Construct sharing URL without using any script
  $twitterURL = 'https://twitter.com/intent/tweet?text=".$crunchifyTitle."&url=".$crunchifyURL."&via=Crunchify';
  $facebookURL = 'https://www.facebook.com/sharer/sharer.php?u='.$crunchifyURL;
  $googleURL = 'https://plus.google.com/share?url=".$crunchifyURL;
  $bufferURL = "https://bufferapp.com/add?url=".$crunchifyURL."&text=".$crunchifyTitle;
  $linkedInURL = "https://www.linkedin.com/shareArticle?mini=true&url=".$crunchifyURL."&title=".$crunchifyTitle;

  // Based on popular demand added Pinterest too
  $pinterestURL = "https://pinterest.com/pin/create/button/?url=".$crunchifyURL."&media=".$crunchifyThumbnail[0]."&description='.$crunchifyTitle;

  // Add sharing button at the end of page/page content
  $variable="<div class="crunchify-social">";
  $variable .= '<a class="crunchify-link crunchify-twitter" href="'. $twitterURL .'" target="_blank">Twitter</a>';
  $variable .= '<a class="crunchify-link crunchify-facebook" href="'.$facebookURL.'" target="_blank">Facebook</a>';
  $variable .= '<a class="crunchify-link crunchify-googleplus" href="'.$googleURL.'" target="_blank">Google+</a>';
  $variable .= '<a class="crunchify-link crunchify-buffer" href="'.$bufferURL.'" target="_blank">Buffer</a>';
  $variable .= '<a class="crunchify-link crunchify-linkedin" href="'.$linkedInURL.'" target="_blank">LinkedIn</a>';
  $variable .= '<a class="crunchify-link crunchify-pinterest" href="'.$pinterestURL.'" data-pin-custom="true" target="_blank">Pin It</a>';
  $variable .= '</div>';

  return $variable.$content;
}