Add custom field to Category and taxonomies

category_add_form_fields.php

<?php
// Add the field to the Add New Category page
add_action( 'category_add_form_fields', 'pt_taxonomy_add_new_meta_field', 10, 2 );
 
function pt_taxonomy_add_new_meta_field() {
    // this will add the custom meta field to the add new term page
    ?>
    <div class="form-field">
        <label for="term_meta[cat_icon]"><?php _e( 'Font Awesome Icons', 'pt' ); ?></label>
        <input type="text" name="term_meta[cat_icon]" id="term_meta[cat_icon]" value="">
        <p class="description"><?php printf(__( 'Choose your category icon from <a href="%s" target="_blank">Font Awesome Icons</a>. For example: <b>fa-wordpress</b>','pt' ), 'http://fontawesome.io/icons/'); ?></p>
    </div>
<?php
}

category_edit_form_fields.php

<?php
// Add the field to the Edit Category page
add_action( 'category_edit_form_fields', 'pt_taxonomy_edit_meta_field', 10, 2 );
 
function pt_taxonomy_edit_meta_field($term) {
 
    // put the term ID into a variable
    $t_id = $term->term_id;
 
    // retrieve the existing value(s) for this meta field. This returns an array
    $term_meta = get_option( "taxonomy_$t_id" ); ?>
    <tr class="form-field">
    <th scope="row" valign="top"><label for="term_meta[cat_icon]"><?php _e( 'Font Awesome Icons', 'pt' ); ?></label></th>
        <td>
            <input type="text" name="term_meta[cat_icon]" id="term_meta[cat_icon]" value="<?php echo esc_attr( $term_meta['cat_icon'] ) ? esc_attr( $term_meta['cat_icon'] ) : ''; ?>">
            <p class="description"><?php printf(__( 'Choose your category icon from <a href="%s" target="_blank">Font Awesome Icons</a>. For example: <b>fa-wordpress</b>','pt' ), 'http://fontawesome.io/icons/'); ?></p>
        </td>
    </tr>
<?php
}

custom_field_usage.php

<?php
	$cat 		= get_the_category();
	$cat_id   	= $cat[0]->term_id;
	$cat_data 	= get_option("taxonomy_$cat_id");?>

	<a href="<?php echo get_category_link($cat[0]->term_id); ?>">
		<?php if (isset($cat_data['cat_icon'])){
			echo '<i class="fa '.$cat_data['cat_icon'].'"></i>';
		}
		?>
		<?php echo $cat[0]->cat_name; ?>
	</a>

save_field_value.php

<?php
// Save extra taxonomy fields callback function.
add_action( 'edited_category', 'pt_save_taxonomy_custom_meta', 10, 2 );
add_action( 'create_category', 'pt_save_taxonomy_custom_meta', 10, 2 );
 
function pt_save_taxonomy_custom_meta( $term_id ) {
    if ( isset( $_POST['term_meta'] ) ) {
        $t_id = $term_id;
        $term_meta = get_option( "taxonomy_$t_id" );
        $cat_keys = array_keys( $_POST['term_meta'] );
        foreach ( $cat_keys as $key ) {
            if ( isset ( $_POST['term_meta'][$key] ) ) {
                $term_meta[$key] = $_POST['term_meta'][$key];
            }
        }
        // Save the option array.
        update_option( "taxonomy_$t_id", $term_meta );
    }
}