[PHP] GD または Imagick で画像をリサイズする

functions.php

<?php
/**
 * 内接サイズを計算する
 *
 * @see https://qiita.com/suin/items/b01eebc05209dba0eb3e
 *
 * @param int $width
 * @param int $height
 * @param int $containerWidth
 * @param int $containerHeight
 *
 * @return array
 */
function get_contain_size( $width, $height, $containerWidth, $containerHeight ) {
	$ratio          = $width / $height;
	$containerRatio = $containerWidth / $containerHeight;

	if ( $ratio > $containerRatio ) {
		$result = array( $containerWidth, intval( $containerWidth / $ratio ) );
	} else {
		$result = array( intval( $containerHeight * $ratio ), $containerHeight );
	}

	return $result;
}

/**
 * リサイズ画像を作成する
 *
 * @param array $file
 * @param array $size
 *
 * @return bool
 */
function make_thumbnail( $file = array(), $size = array() ) {
	$library = determination_image_library();
	$result  = false;

	if ( $library === 'gd' ) {
		$result = resize_image_using_gd( $file, $size );
	} elseif ( $library === 'imagick' ) {
		$result = resize_image_using_imagick( $file, $size );
	}

	return $result;
}

/**
 * 画像処理ライブラリの判定
 * 判定は GD が優先される
 *
 * @return null|string
 */
function determination_image_library () {
	$result = null;

	if ( extension_loaded( 'gd' ) && function_exists( 'gd_info' ) ) {
		$result = 'gd';
	} elseif ( extension_loaded( 'imagick' ) && class_exists( 'Imagick' ) ) {
		$result = 'imagick';
	}

	return $result;
}

/**
 * GD ライブラリを使って画像をリサイズする
 *
 * @param array $file
 * @param array $size
 *
 * @return bool
 */
function resize_image_using_gd( $file = array(), $size = array() ) {
	$result = false;

	switch ( $file['type'] ) {
		case IMAGETYPE_JPEG:
			$source = imagecreatefromjpeg( $file['src'] );
			break;
		case IMAGETYPE_PNG:
			$source = imagecreatefrompng( $file['src'] );
			break;
		case IMAGETYPE_GIF:
			$source = imagecreatefromgif( $file['src'] );
			break;
		default:
			$source = null;
	}

	if ( ! is_null( $source ) ) {
		$canvas = imagecreatetruecolor( $size['width'], $size['height'] );

		imagecopyresampled( $canvas, $source, 0, 0, 0, 0, $size['width'], $size['height'], $size['org_width'], $size['org_height'] );
		imagejpeg( $canvas, $file['dest'] );
		imagedestroy( $source );
		imagedestroy( $canvas );

		$result = true;
	}

	return $result;
}

/**
 * Imagick ライブラリを使って画像をリサイズする
 *
 * @param array $file
 * @param array $size
 *
 * @return bool
 */
function resize_image_using_imagick( $file = array(), $size = array() ) {
	$result = false;

	switch ( $file['type'] ) {
		case IMAGETYPE_JPEG:
			$ext = 'JPG';
			break;
		case IMAGETYPE_PNG:
			$ext = 'PNG';
			break;
		case IMAGETYPE_GIF:
			$ext = 'GIF';
			break;
		default:
			$ext = null;
	}

	if ( ! is_null( $ext ) ) {
		$image = new Imagick( $file['src'] );
		$image->stripImage();
		$image->scaleImage( $size['width'], $size['height'] );
		$image->setImageFormat( $ext );
		$image->setImageCompressionQuality( 90 );
		$image->writeImage( $file['dest'] );
		$image->destroy();

		$result = true;
	}

	return $result;
}

index.php

<?php
list( $org_width, $org_height, $type ) = getimagesize( $src_file );
list( $resize_width, $resize_height ) = get_contain_size( $org_width, $org_height, '480', '270' );
$ext  = pathinfo( $src_file, PATHINFO_EXTENSION );
$file = array(
  'src'  => $src_file,
  'dest' => str_replace( ".{$ext}", "", $src_file ) . "-480.{$ext}",
  'type' => $type,
);
$size = array(
  'width'      => $resize_width,
  'height'     => $resize_height,
  'org_width'  => $org_width,
  'org_height' => $org_height,
);

make_thumbnail( $file, $size );