我正在尝试在Laravel Project中安装作曲器，但会引发此错误。
我在ubuntu18.04中安装了php7.0，并且成功安装了composer，但是项目无法正常工作。
Fatal error: Uncaught Error: Call to undefined function Composer\Json\json_decode() in /usr/share/php/Composer/Json/JsonFile.php:153
Stack trace:
#0 /usr/share/php/Composer/Factory.php(290): Composer\Json\JsonFile->validateSchema(1)
#1 /usr/share/php/Composer/Factory.php(576): Composer\Factory->createComposer(Object(Composer\IO\ConsoleIO), './composer.json', false)
#2 /usr/share/php/Composer/Console/Application.php(338): Composer\Factory::create(Object(Composer\IO\ConsoleIO), NULL, false)
#3 /usr/share/php/Composer/Console/Application.php(451): Composer\Console\Application->getComposer(false, false)
#4 /usr/share/php/Composer/Console/Application.php(149): Composer\Console\Application->getPluginCommands()
#5 /usr/share/php/Symfony/Component/Console/Application.php(148): Composer\Console\Application->doRun(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#6 /usr/share/php/Composer/Console/Application.php(100): Symfony\Component\Console\Application->run(Object(Symfony in /usr/share/php/Composer/Json/JsonFile.php on line 153