Adds image size to WordPress with setting to control the dimensions in media.php screen and size name in WP user interface.

PostImageSize.php

<?php
/**
 * Adds image size to WordPress with setting to control the dimensions in media.php screen and size name in WP user interface.
 * example:
 *
 * if( !defined('PIS_TXD') ) {
 *      define( 'PIS_TXD', 'your-text-domain' );
 * }
 * require_once( dirname( __FILE__ ) . '/PostImageSize.php' );
 * $teaser_size = new PostImageSize( 'teaser', __( 'Teaser', 'your-text-domain' ), 460, 280, true );
 *
 *
 * @file PostImageSize.php
 * @copyright (c) 2017 Serge Liatko <[email protected]> https://sergeliatko.com
 */
/** load class only if not previously loaded */
if( !class_exists('PostImageSize') ) {
	/**
	 * Class PostImageSize
	 */
	class PostImageSize {

		/** @var  string $name image size name */
		public $name;

		/** @var  string $title image size title */
		public $title;

		/** @var  int $width image width */
		public $width;

		/** @var  int $height image height */
		public $height;

		/** @var  bool $crop image hard crop */
		public $crop;

		/**
		 * PostImageSize constructor.
		 *
		 * @param string $name
		 * @param string $title
		 * @param int $width
		 * @param int $height
		 * @param bool $crop
		 */
		public function __construct( $name = 'thumbnail', $title, $width = 125, $height = 125, $crop = false ) {

			/** define text domain */
			if( !defined('PIS_TXD') ) {
				define( 'PIS_TXD', 'post-image-size' );
			}

			$title = empty( $title ) ? $name : $title;

			$this->setName( $name );
			$this->setTitle( $title );
			$this->setWidth( $width );
			$this->setHeight( $height );
			$this->setCrop( $crop );

			/** add image size to WP */
			add_action( 'after_setup_theme', array( $this, 'add_image_size' ), 10, 0 );

			/** add image size name to drop-down of names */
			add_filter( 'image_size_names_choose', array( $this, 'image_size_names_choose' ), 10, 1 );

			/** add size user interface */
			add_action( 'admin_init', array( $this, 'add_setting_field' ), 10, 0 );

			/** register setting */
			add_action( 'admin_init', array( $this, 'register_setting' ), 10, 0 );
		}

		/**
		 * Displays setting field
		 */
		public function setting_field() {
			printf(
				'<legend class="screen-reader-text"><span>%1$s</span></legend>%2$s %3$s<br />%4$s',
				sprintf( __( '%1$s size', PIS_TXD ), $this->getTitle() ),
				$this->get_width_field(),
				$this->get_height_field(),
				$this->get_crop_field()
			);
		}

		/**
		 * Returns setting field width HTML.
		 *
		 * @return string
		 */
		protected function get_width_field() {
			return sprintf(
				'<label for="%2$s">%1$s</label> <input id="%2$s" name="%3$s" type="number" value="%4$s" class="small-text" min="0" step="1" />',
				( $this->isCrop() ? __( 'Width', PIS_TXD ) : __( 'Max Width', PIS_TXD ) ),
				sprintf( '%1$s-size-w', $this->getName() ),
				sprintf( '%1$s_size_w', $this->getName() ),
				esc_attr( $this->getWidth() )
			);
		}

		/**
		 * Returns setting field height HTML.
		 *
		 * @return string
		 */
		protected function get_height_field() {
			return sprintf(
				'<label for="%2$s">%1$s</label> <input id="%2$s" name="%3$s" type="number" value="%4$s" class="small-text" min="0" step="1" />',
				( $this->isCrop() ? __( 'Height', PIS_TXD ) : __( 'Max Height', PIS_TXD ) ),
				sprintf( '%1$s-size-h', $this->getName() ),
				sprintf( '%1$s_size_h', $this->getName() ),
				esc_attr( $this->getHeight() )
			);
		}

		/**
		 * Returns setting field crop HTML.
		 *
		 * @return string
		 */
		protected function get_crop_field() {
			return sprintf(
				'<input id="%2$s" name="%3$s" type="checkbox" value="1" %4$s/> <label for="%2$s">%1$s</label>',
				__( 'Crop image to exact dimensions (leave empty to keep images proportional)', PIS_TXD ),
				sprintf( '%1$s-crop', $this->getName() ),
				sprintf( '%1$s_crop', $this->getName() ),
				checked( true, $this->isCrop(), false )
			);
		}

		/**
		 * Registers setting field in media options.
		 */
		public function add_setting_field() {
			add_settings_field(
				$this->getName() . '_size',
				sprintf( __( '%1$s size', PIS_TXD ), $this->getTitle() ),
				array( $this, 'setting_field' ),
				'media',
				'default'
			);
		}

		/**
		 * Registers setting for saving.
		 */
		public function register_setting() {
			register_setting( 'media', $this->getName() . '_size_w', 'absint' );
			register_setting( 'media', $this->getName() . '_size_h', 'absint' );
			register_setting( 'media', $this->getName() . '_crop', 'absint' );
		}

		/**
		 * Adds image size to WP.
		 */
		public function add_image_size() {
			add_image_size( $this->getName(), $this->getWidth(), $this->getHeight(), $this->isCrop() );
		}

		/**
		 * Adds image size name to drop-down of available image sizes.
		 *
		 * @param array $sizes
		 *
		 * @return array
		 */
		public function image_size_names_choose( $sizes = array() ) {
			return array_merge( $sizes, array( $this->getName() => $this->getTitle() ) );
		}

		/**
		 * Returns option from database or default if the option is empty.
		 *
		 * @param string $option
		 * @param mixed $default
		 *
		 * @return mixed
		 */
		protected function get_not_empty_option( $option, $default ) {
			return $this->is_empty( $value = get_option( $option, $default ) ) ? $default : $value;
		}

		/**
		 * Check if data is empty.
		 *
		 * @param mixed $data
		 *
		 * @return bool
		 */
		protected function is_empty( $data ) {
			return empty( $data );
		}

		/**
		 * @return string
		 */
		public function getName() {
			return $this->name;
		}

		/**
		 * @param string $name
		 */
		public function setName( $name ) {
			$this->name = $name;
		}

		/**
		 * @return string
		 */
		public function getTitle() {
			return $this->title;
		}

		/**
		 * @param string $title
		 */
		public function setTitle( $title ) {
			$this->title = $title;
		}

		/**
		 * @return int
		 */
		public function getWidth() {
			return absint( $this->get_not_empty_option( $this->getName() . '_size_w', $this->width ) );
		}

		/**
		 * @param int $width
		 */
		public function setWidth( $width ) {
			$this->width = $width;
		}

		/**
		 * @return int
		 */
		public function getHeight() {
			return absint( $this->get_not_empty_option( $this->getName() . '_size_h', $this->height ) );
		}

		/**
		 * @param int $height
		 */
		public function setHeight( $height ) {
			$this->height = $height;
		}

		/**
		 * @return bool
		 */
		public function isCrop() {
			return !$this->is_empty( get_option( $this->getName() . '_crop', $this->crop ) );
		}

		/**
		 * @param bool $crop
		 */
		public function setCrop( $crop ) {
			$this->crop = $crop;
		}
	}
}