如果在代码中使用提示：imagecreatefrompng(): is not a valid PNG file，表示需要安装jpeg库。

直接运行函数，提示：

[root@server gd]# php -r 'imagecreatefromjpeg();'
PHP Fatal error:  Uncaught Error: Call to undefined function imagecreatefromjpg() in Command line code:1
Stack trace:
#0 {main}
  thrown in Command line code on line 1

Fatal error: Uncaught Error: Call to undefined function imagecreatefromjpg() in Command line code:1
Stack trace:
#0 {main}
  thrown in Command line code on line 1

或者检查：php --re=gd | grep jpeg

下载jpeg拓展库

[root@server ~]# wget http://www.ijg.org/files/jpegsrc.v9c.tar.gz
[root@server ~]# tar xvf jpegsrc.v9c.tar.gz
[root@server ~]# cd jpeg-9c/

编译jpeg库：

[root@server jpeg-9c]# ./configure --prefix=/usr/local/phpjpeglib --enable-shared --enable-static 

进入php的gd库拓展目录：

[root@server ~]# cd /root/php-7.4.10/ext/gd

找到phpize路径：
一般在/usr/local/php/bin/phpize或者/usr/bin/phpize

[root@server gd]# /usr/local/php/bin/phpize
Configuring for:
PHP Api Version:         20190902
Zend Module Api No:      20190902
Zend Extension Api No:   320190902

找到php-config路径/usr/local/php/bin/php-config

在gd目录（非php源码根目录），只需要使用新的配置编译（关键点是！必须使用php-config选项）：

[root@server ~]# cd /root/php-7.4.10/ext/gd
[root@server ~]# ./configure --with-php-config=/usr/local/php/bin/php-config --with-jpeg=/usr/local/phpjpeglib  --with-freetype=/usr/local/freetype

[root@server gd]# make && make install

以下表示按安装成功：

/bin/sh /root/php-7.4.10/ext/gd/libtool --mode=install cp ./gd.la /root/php-7.4.10/ext/gd/modules
cp ./.libs/gd.so /root/php-7.4.10/ext/gd/modules/gd.so
cp ./.libs/gd.lai /root/php-7.4.10/ext/gd/modules/gd.la
PATH="$PATH:/sbin" ldconfig -n /root/php-7.4.10/ext/gd/modules
----------------------------------------------------------------------
Libraries have been installed in:
   /root/php-7.4.10/ext/gd/modules

If you ever happen to want to link against installed libraries
in a given directory, LIBDIR, you must either use libtool, and
specify the full pathname of the library, or use the `-LLIBDIR'
flag during linking and do at least one of the following:
   - add LIBDIR to the `LD_LIBRARY_PATH' environment variable
     during execution
   - add LIBDIR to the `LD_RUN_PATH' environment variable
     during linking
   - use the `-Wl,--rpath -Wl,LIBDIR' linker flag
   - have your system administrator add LIBDIR to `/etc/ld.so.conf'

See any operating system documentation about shared libraries for
more information, such as the ld(1) and ld.so(8) manual pages.
----------------------------------------------------------------------

Build complete.
Don't forget to run 'make test'.

Installing shared extensions:     /usr/local/php/lib/php/extensions/no-debug-zts-20190902/
Installing header files:          /usr/local/php/include/php/

最后在php.ini文件启用gd拓展（注意php.ini是不是你正在使用的ini），然后重启php-fpm，完成！

[root@server gd]# php --re=gd | grep jpeg
    Entry [ gd.jpeg_ignore_warning <ALL> ]
    Function [ <internal:gd> function imagecreatefromjpeg ] {
    Function [ <internal:gd> function imagejpeg ] {
    Function [ <internal, deprecated:gd> function jpeg2wbmp ] {

温馨提示

这里的不是全部重新编译，使用phpize，之前已经编译完成的模块等，不会受到一丁点影响。

验证码需要用到 freetype 所以gd的选项，要一起编译。

题外话

我centos 8安装的时候，总是不成功，后面鬼使神差在php目录编译了一下：
[root@server gd]# ./configure --with-php-config=/usr/local/php/bin/php-config
然后再进入gd编译，可以了。。。
有一些环境，只能够通过重新编译php解决，暂时找不到原因，php7.4.10参考编译参数：

./configure  --prefix=/usr/local/php --with-config-file-path=/usr/local/php/etc --enable-fpm --with-fpm-user=www --with-fpm-group=www --enable-mysqlnd --with-mysqli=mysqlnd --with-pdo-mysql=mysqlnd --enable-mysqlnd-compression-support --with-iconv-dir --with-zlib --enable-xml --disable-rpath --enable-bcmath --enable-shmop --enable-sysvsem --enable-inline-optimization --with-curl --enable-mbregex --enable-mbstring --enable-intl --with-openssl --with-mhash --enable-pcntl --enable-sockets --with-xmlrpc --enable-soap --with-gettext --disable-fileinfo --enable-opcache --with-pear --enable-maintainer-zts --with-ldap=shared --without-gdbm --enable-pcntl --enable-gd --enable-fileinfo --with-jpeg=/usr/local/phpjpeglib --with-freetype=/usr/local/freetype

哎，还是对编译安装不够精通。