Some commonly used functions in WordPress theme development.

wp_functions.php

<?php

if (!defined('ABSPATH')) exit("Direct access not permitted.");

define('APP_NAME', 'Viral Startup');
define('APP_SLUG', 'viral-startup');
define('APP_VERSION', SCRIPT_DEBUG ? uniqid() : 1);
define('ASSETS_PATH', get_template_directory() . '/assets/dist');
define('ASSETS_URI', get_template_directory_uri() . '/assets/dist');

require_once 'vendor/autoload.php';
require_once 'wp_helpers.php';
#
# SET PHP TIMEZONE TO MATCH WP SETTINGS
#

$timezone_string = get_option('timezone_string');
if (empty($timezone_string)) $timezone_string = 'UTC';
date_default_timezone_set($timezone_string);

#
# REGISTER WEB ASSETS
#

function remove_default_styles()
{
    global $wp_styles;
    foreach ($wp_styles->registered as $handle => $data) {
        wp_deregister_style($handle);
        wp_dequeue_style($handle);
    }
}

add_action('wp_enqueue_scripts', 'remove_default_styles', 99);

function remove_default_scripts()
{
    global $wp_scripts;
    foreach ($wp_scripts->registered as $handle => $data) {
        wp_deregister_script($handle);
        wp_dequeue_script($handle);
    }
}

add_action('wp_enqueue_scripts', 'remove_default_scripts', 99);

function enqueue_theme_styles()
{
    wp_enqueue_style('normalize', 'https://cdnjs.cloudflare.com/ajax/libs/normalize/6.0.0/normalize.min.css', null, null);
    wp_enqueue_style('flatpickr', 'https://cdnjs.cloudflare.com/ajax/libs/flatpickr/2.4.9/flatpickr.min.css', null, null);
    wp_enqueue_style('pannellum', 'https://cdnjs.cloudflare.com/ajax/libs/pannellum/2.3.2/pannellum.css', null, null);

    wp_enqueue_style(
        APP_SLUG . '-styles',
        ASSETS_URI . '/css/app' . (SCRIPT_DEBUG ? '.css' : '.min.css'),
        null,
        APP_VERSION
    );
}

add_action('wp_enqueue_scripts', 'enqueue_theme_styles', 100);

function enqueue_theme_scripts()
{
    wp_enqueue_script('vue', 'https://cdnjs.cloudflare.com/ajax/libs/vue/2.2.6/vue.min.js', null, null, true);
    wp_enqueue_script('flatpickr', 'https://cdnjs.cloudflare.com/ajax/libs/flatpickr/2.4.9/flatpickr.min.js', null, null, true);
    wp_enqueue_script('pannellum', 'https://cdnjs.cloudflare.com/ajax/libs/pannellum/2.3.2/pannellum.js', null, null, true);

    wp_enqueue_style(
        APP_SLUG . '-scripts',
        ASSETS_URI . '/js/app' . (SCRIPT_DEBUG ? '.js' : '.min.js'),
        null,
        APP_VERSION,
        true
    );

    if (WP_DEBUG) {
        $url = 'http://127.0.0.1:35729/livereload.js';
        if (url_exists($url)) {
            wp_enqueue_script(APP_SLUG . '-livereload', $url, null, APP_VERSION, true);
        }
    }

    $data = [
        'appName' => APP_NAME,
        'appVersion' => APP_VERSION,
        'assetsUrl' => ASSETS_URI,
        'ajaxUrl' => admin_url('admin-ajax.php'),
    ];

    wp_localize_script(APP_SLUG . '-scripts', 'WP', $data);
}

add_action('wp_enqueue_scripts', 'enqueue_theme_scripts', 100);

function enqueue_admin_styles()
{
    wp_enqueue_style(
        APP_SLUG . '-admin-styles',
        ASSETS_URI . '/css/admin' . (SCRIPT_DEBUG ? '.css' : '.min.css'),
        null,
        APP_VERSION
    );
}

add_action('login_enqueue_scripts', 'enqueue_admin_styles');
add_action('admin_enqueue_scripts', 'enqueue_admin_styles');

#
# ADD FAVICON TO ADMIN
#

function add_favicon()
{
    $favicon_url = ASSETS_URI . '/favicon.ico';
    $favicon_markup = sprintf('<link rel="shortcut icon" href="%s" />', $favicon_url);
    echo $favicon_markup;
}

add_action('login_head', 'add_favicon');
add_action('admin_head', 'add_favicon');

#
# ALLOW SVG UPLOADS
#

function custom_upload_mimes($mimes)
{
    $mimes['svg'] = 'image/svg+xml';
    return $mimes;
}

add_filter('upload_mimes', 'custom_upload_mimes');

#
# REMOVE MENU PAGES
#

function remove_comments_menu()
{
    remove_menu_page('edit-comments.php');
}

function remove_acf_menu()
{
    if (!WP_DEBUG) {
        remove_menu_page('edit.php?post_type=acf-field-group');
    }
}

add_action('admin_menu', 'remove_comments_menu', 999);
add_action('admin_menu', 'remove_acf_menu', 999);

#
# DISABLE ADMIN BAR
#

add_filter('show_admin_bar', '__return_false');

#
# REMOVE FLUFF
#

// Remove WordPress emoji scripts
remove_action('wp_head', 'print_emoji_detection_script');
remove_action('wp_print_styles', 'print_emoji_styles');

// Remove WordPress canonical links
remove_action('wp_head', 'rel_canonical');

#
# CUSTOM EXCERPT
#

function custom_excerpt_length()
{
    return 75;
}

add_filter('excerpt_length', 'custom_excerpt_length', 999);

function custom_excerpt_more()
{
    return '...';
}

add_filter('excerpt_more', 'custom_excerpt_more');

#
# CUSTOM EDITOR STYLESHEET
#

function custom_editor_stylesheet()
{
    add_editor_style('style.css');
}

add_action('admin_init', 'custom_editor_stylesheet');

#
# CUSTOM BODY CLASSES
#

function custom_body_classes($classes)
{
    global $post;
    if ($post) $classes[] = "page-{$post->post_name}";
    return $classes;
}

add_filter('body_class', 'custom_body_classes');

wp_helpers.php

<?php

/**
 * Embed an SVG
 * @param $filename string
 * @param $classes array
 * @param $sizes array|null
 * @return string
 */
function embed_svg($filename, array $classes = [], array $sizes = null)
{
    $path = get_image_path("{$filename}.svg");
    $svg = file_get_contents($path);
    $classes[] = 'svg-container';
    $classes[] = $filename;
    $attributes = [];
    $attributes['class'] = implode(' ', $classes);
    if ($sizes) {
        $width = $sizes[0];
        $height = $sizes[1] ?? $width;
        $attributes['style'] = implode('; ', [
            "width:{$width}",
            "height:{$height}",
        ]);
    }
    $attributes = implode(
        array_map(
            function ($value, $key) {
                return sprintf('%s="%s"', $key, $value);
            },
            array_values($attributes),
            array_keys($attributes)
        ),
        ' '
    );
    return "<span {$attributes}>{$svg}</span>";
}

/**
 * Function used to create a url friendly slug from any string
 * @param $text string The string to convert into a slug
 * @return string
 */
function slugify($text)
{
    // replace non letter or digits by -
    $text = preg_replace('~[^\pL\d]+~u', '-', $text);
    // transliterate
    $text = iconv('utf-8', 'us-ascii//TRANSLIT', $text);
    // remove unwanted characters
    $text = preg_replace('~[^-\w]+~', '', $text);
    // trim
    $text = trim($text, '-');
    // remove duplicate -
    $text = preg_replace('~-+~', '-', $text);
    // lowercase
    $text = strtolower($text);
    return $text;
}

/**
 * Apply a character limit to a string
 * @param string $str String to get an excerpt from
 * @param integer $startPos Position int string to start excerpt from
 * @param integer $maxLength Maximum length the excerpt may be
 * @return string
 */
function char_limit($str, $startPos = 0, $maxLength = 160)
{
    $excerpt = $str;
    if (strlen($str) > $maxLength) {
        $excerpt = substr($str, $startPos, $maxLength - 3);
        $lastSpace = strrpos($excerpt, ' ');
        $excerpt = substr($excerpt, 0, $lastSpace);
        $excerpt .= '...';
    }
    return $excerpt;
}

/**
 * Apply a word limit to a string
 * @param string $str The string to get an excerpt from
 * @param integer $limit The number of words to allow
 * @param string $after A string to append at the end
 * @return string
 */
function word_limit($str, $limit = 20, $after = '...')
{
    if (str_word_count($str, 0) > $limit) {
        $words = str_word_count($str, 2);
        $pos = array_keys($words);
        $str = substr($str, 0, $pos[$limit]) . $after;
    }
    return $str;
}

/**
 * Generates a human readable timestamp relative to the current time
 * @param integer $timestamp The input time used to generate a timestamp relative to now
 * @param string $format The date format used when the timestamp provided exceeds a month
 * @return string
 */
function relative_timestamp($timestamp, $format = 'M j, Y')
{
    if (!ctype_digit($timestamp)) {
        $timestamp = strtotime($timestamp);
    }
    $diff = time() - $timestamp;
    if ($diff == 0) {
        return 'Now';
    } elseif ($diff > 0) {
        $day_diff = floor($diff / 86400);
        if ($day_diff == 0) {
            if ($diff < 60) return 'Just now';
            if ($diff < 120) return '1 minute ago';
            if ($diff < 3600) return floor($diff / 60) . ' minutes ago';
            if ($diff < 7200) return '1 hour ago';
            if ($diff < 86400) return floor($diff / 3600) . ' hours ago';
        }
        if ($day_diff == 1) return 'Yesterday';
        if ($day_diff < 7) return $day_diff . ' days ago';
        if ($day_diff < 31) return ceil($day_diff / 7) . ' weeks ago';
        if ($day_diff < 60) return 'Last month';
        return date($format, $timestamp);
    } else {
        $diff = abs($diff);
        $day_diff = floor($diff / 86400);
        if ($day_diff == 0) {
            if ($diff < 120) return 'In a minute';
            if ($diff < 3600) return 'In ' . floor($diff / 60) . ' minutes';
            if ($diff < 7200) return 'In an hour';
            if ($diff < 86400) return 'In ' . floor($diff / 3600) . ' hours';
        }
        if ($day_diff == 1) return 'Tomorrow';
        if ($day_diff < 4) return date('l', $timestamp);
        if ($day_diff < 7 + (7 - date('w'))) return 'Next week';
        if (ceil($day_diff / 7) < 4) return 'In ' . ceil($day_diff / 7) . ' weeks';
        if (date('n', $timestamp) == date('n') + 1) return 'Next month';
        return date($format, $timestamp);
    }
}