PHP 画像を正方形の枠いっぱいにリサイズするには？

curl -LO https://answorz.com/10025/Image.php

<?php
declare(strict_types=1);

class Image
{
  private $resource = null;
  private int $bgcolor;

  public function __construct( string $filepath = '' )
  {
    if ('' != $filepath)
      $this->open($filepath);
    $this->set_bgcolor_transparent();
  }

  public function __destruct()
  {
    $this->close();
  }

  public function create( int $width, int $height ):self
  {
    $this->resource = imagecreatetruecolor($width,$height);
    return $this;
  }

  // 画像を開く
  public function open( string $filepath ):self
  {
    $this->close();

    $this->resource = null;

    if (false !== getimagesize($filepath)) {
      exif_imagetype($filepath);
      $resource = match (exif_imagetype($filepath)) {
        1 => imageCreateFromGif($filepath),
        2 => imageCreateFromJpeg($filepath),
        3 => imageCreateFromPng($filepath),
        6 => imageCreateFromBmp($filepath),
      };
      if (!is_null($resource)) {
        $this->resource = $resource;
      }
    }
    return $this;
  }

  public function close():self
  {
    if (!is_null($this->resource))
      imagedestroy($this->resource);
    return $this;
  }

  public function getResource()
  {
    return $this->resource;
  }

  // 画像を保存
  public function save( string $filepath, int $quality = 100 ):self
  {
    $png_quality = intval(round(($quality - 100) / 11.111111));
    match (pathinfo($filepath,PATHINFO_EXTENSION)) {
      'jpeg','jpg' => imagejpeg($this->resource,$filepath,$quality),
      'png'        => imagepng($this->resource,$filepath,$png_quality),
      'gif'        => imagegif($this->resource,$filepath),
      'bmp'        => imagebmp($this->resource,$filepath),
    };
    return $this;
  }

  // 画像を出力
  public function __toString():string
  {
    ob_start();
    imagejpeg($this->resource,null,100);
    $data = ob_get_contents();
    ob_end_clean();
    return ('' == $data) ? '' : 'data:image/jpeg;base64,'.base64_encode($data);
  }

  // 画像をリサイズ（正方形）
  public function resize_square( $length ):self
  {
    $left = $top = 0;
    if ($this->get_width() > $this->get_height()) {
      $this->resize($length,0);
      $top = intval(($length - $this->get_height()) / 2);
    }
    else {
      $this->resize(0,$length);
      $left = intval(($length - $this->get_width()) / 2);
    }
    $width = $this->get_width();
    $height = $this->get_height();
    $old_resource = $this->resource;
    $this->create($length,$length)->fill($this->bgcolor);
    imagecopy($this->resource,$old_resource,$left,$top,0,0,$width,$height);
    imagedestroy($old_resource);
    return $this;
  }

  // 画像をリサイズ
  public function resize( int $width = 0, int $height = 0 ):self
  {
    $old_width = $this->get_width();
    $old_heigth = $this->get_height();
    if (0 == $width && 0 != $height) {
      $width  = intval(round(($height / $old_heigth) * $old_width));
    }
    else if (0 == $height && 0 != $width) {
      $height = intval(round(($width / $old_width) * $old_heigth));
    }
    $old_resource = $this->resource;
    $this->create($width,$height);
    imagecopyresampled($this->resource,$old_resource,0,0,0,0,$width,$height,$old_width,$old_heigth);
    imagedestroy($old_resource);
    return $this;
  }

  // 画像を切り抜く
  public function trim( int $top = 0, int $right = 0, int $bottom = 0, int $left = 0 ):self
  {
    $width = $this->get_width() - $left - $right;
    $height = $this->get_height() - $top - $bottom;
    $old_resource = $this->resource;
    $this->create($width,$height);
    imagecopy($this->resource,$old_resource,0,0,$left,$top,$width,$height);
    imagedestroy($old_resource);
    return $this;
  }

  // 画像をコピー
  public function copy( Image $img, int $dx = 0 , int $dy =0 ):self
  {
    imagecopy($this->resource,$img->get_resource(),$dx,$dy,0,0,$img->get_width(),$img->get_height());
    return $this;
  }

  // 画像を回転
  public function rotate( float $angle ):self
  {
    $old_resource = $this->resource;
    $this->resource = imagerotate($this->resource,$angle,$this->bgcolor,0);
    imagedestroy($old_resource);
    return $this;
  }

  // 画像を反転
  public function flip( int $mode ):self
  {
    imageflip($this->resource,$mode);
    return $this;
  }

  // 画像を塗りつぶす
  public function fill( int $color ):self
  {
    imagefill($this->resource,0,0,$color);
    return $this;
  }

  public function get_resource()
  {
    return $this->resource;
  }

  public function get_width():int
  {
    return imagesx($this->resource);
  }

  public function get_height():int
  {
    return imagesy($this->resource);
  }

  public function set_bgcolor( int $bgcolor ):self
  {
    $this->bgcolor = $bgcolor;
    return $this;
  }

  // 背景を透明にする
  public function set_bgcolor_transparent():self
  {
    $this->bgcolor = imagecolorallocatealpha($this->resource,0,0,0,127);
    return $this;
  }

  // 背景を白にする
  public function set_bgcolor_white():self
  {
    $this->bgcolor = imagecolorallocatealpha($this->resource,255,255,255,0);
    return $this;
  }

  // 背景を黒にする
  public function set_bgcolor_black():self
  {
    $this->bgcolor = imagecolorallocatealpha($this->resource,0,0,0,0);
    return $this;
  }

  public function get_bgcolor():int
  {
    return $this->bgcolor;
  }
}

<?php
include 'Image.php';
$img = (new Image('mosss.png'))->set_bgcolor_white()->resize_square(600)->save('mosss2.png');
?>
<html>
<body>
<img src="<?= $img ?>">
</body>
</html>