[WordPress] Programmatically Add Post Terms in WordPress

1-set-post-term.php

<?php

/**
  * Appends the specified taxonomy term to the incoming post object. If 
 * the term doesn't already exist in the database, it will be created.
 *
 * @param    WP_Post    $post        The post to which we're adding the taxonomy term.
 * @param    string     $value       The name of the taxonomy term
 * @param    string     $taxonomy    The name of the taxonomy.
 * @access   private
 * @since    1.0.0
 */
private function set_post_term( $post, $value, $taxonomy ) {

	$term = term_exists( $value, $taxonomy );

	// If the taxonomy doesn't exist, then we create it
	if ( 0 === $term || null === $term ) {

		$term = wp_insert_term(
			$value,
			$taxonomy,
			array(
				'slug' => strtolower( str_ireplace( ' ', '-', $value ) )
			)
		);

	}

	// Then we can set the taxonomy
	wp_set_post_terms( $post['ID'], $term, $taxonomy, true );

}

2-check-for-multiple-terms.php

<?php

/**
 * Determines if the given value has multiple terms by checking to see
 * if a semicolon exists in the value.
 *
 * @param    string   $value    The value to evaluate for multiple terms.
 * @return   bool               True if there are multiple terms; otherwise, false.
 */
private function has_multiple_terms( $value ) {
	return 0 < strpos( $value, ';' );
}

3-apply-multiple-terms.php

<?php

/**
 * Loops through each of the multiple terms that exist and use the
 * set_profile_terms function to apply each value to the given post
 *
 * @param    WP_Post    $post        The post which we're applying the terms.
 * @param    string     $values      The delimited list of terms.
 * @param    string     $taxonomy    The taxonomy to which the terms belong.
 */
private function set_multiple_terms( $post, $values, $taxonomy ) {

	$terms = explode( ';', $values );
	foreach( $terms as $term ) {
		$this->set_post_terms( $post, $term, $taxonomy );
	}

}

4-expand-original-routine.php

<?php

/**
 * Appends the specified taxonomy term to the incoming post object. If 
 * the term doesn't already exist in the database, it will be created. This
 * function now supports multiple terms.
 * 
 * @param    WP_Post    $post        The current post with which we're working
 * @param    string     $value       The term value (or term name)
 * @param    string     $taxonomy    The name of the taxonomy to which the term belongs.
 * @access   private
 * @since    1.0.0
 */
private function set_post_terms( $post, $value, $taxonomy ) {

	/* First check to see if there are multiple terms and,
	 * if so, then loop through the values and update each
	 * term. 
	 */
	if ( $this->has_multiple_terms( $value ) ) {
		$this->set_multiple_terms( $post, $value, $taxonomy );
	} else {

		$term = term_exists( strtolower( $value ), $taxonomy );

		// If the taxonomy doesn't exist, then we create it
		if ( 0 === $term || null === $term ) {

			$term = wp_insert_term(
				$value,
				$taxonomy,
				array(
					'slug' => strtolower( str_ireplace( ' ', '-', $value ) )
				)
			);

		}
	}

	// Then we can set the taxonomy
	wp_set_post_terms( $post['ID'], $term, $taxonomy, true );

}