1. 安装 homebrew

2. 确定已安装 pkg-config

$ where pkg-config
/usr/local/bin/pkg-config

若提示

pkg-config not found

则先使用 brew install pkg-config 进行安装

1. 安装最新的 OpenSSL

macOS 默认的 OpenSSL 版本太低，编译时需要版本高于1.0.1才可，so just install the lastest one.

$ brew install openssl
==> Downloading https://homebrew.bintray.com/bottles/openssl-1.0.2l.sierra.bottle.tar.gz
Already downloaded: /Users/mrasong/Library/Caches/Homebrew/openssl-1.0.2l.sierra.bottle.tar.gz
==> Pouring openssl-1.0.2l.sierra.bottle.tar.gz
==> Using the sandbox
==> Caveats
A CA file has been bootstrapped using certificates from the SystemRoots
keychain. To add additional certificates (e.g. the certificates added in
the System keychain), place .pem files in
  /usr/local/etc/openssl/certs

and run
  /usr/local/opt/openssl/bin/c_rehash

This formula is keg-only, which means it was not symlinked into /usr/local,
because Apple has deprecated use of OpenSSL in favor of its own TLS and crypto libraries.

If you need to have this software first in your PATH run:
  echo 'export PATH="/usr/local/opt/openssl/bin:$PATH"' >> ~/.zshrc

For compilers to find this software you may need to set:
    LDFLAGS:  -L/usr/local/opt/openssl/lib
    CPPFLAGS: -I/usr/local/opt/openssl/include
For pkg-config to find this software you may need to set:
    PKG_CONFIG_PATH: /usr/local/opt/openssl/lib/pkgconfig

==> Summary
  /usr/local/Cellar/openssl/1.0.2l: 1,709 files, 12.2MB

安装完成后，需要给环境变量里面加入一些东西，上面的提示已经说的很清楚。

vi ~/.zshrc 或者 vi ~/.bash_profile

添加以下内容

export PATH=/usr/local/opt/openssl/bin:$PATH
export LDFLAGS="-L/usr/local/opt/openssl/lib"
export CPPFLAGS="-I/usr/local/opt/openssl/include"
export PKG_CONFIG_PATH="/usr/local/opt/openssl/lib/pkgconfig"

source ~/.zshrc 或者 source ~/.bash_profile

1. pecl 安装 mongodb.so

pecl install mongodb

1. 修改 php.ini，加入 extension=mongodb.so，重启服务，完成。