我跟着这些说明并尝试安装可切换的 PHP 版本。但切换到 5.6 后，网页无法正常工作，我只看到以下内容：
但当我再次切换回 7.0 时，一切都正常。
哪里有问题？
错误日志：
[Fri Sep 14 06:26:01.989335 2018] [mpm_prefork:notice] [pid 9621] AH00163: Apache/2.4.18 (Ubuntu) OpenSSL/1.0.2g configured -- resuming normal operations
[Fri Sep 14 06:26:01.989378 2018] [core:notice] [pid 9621] AH00094: Command line: '/usr/sbin/apache2'
[Fri Sep 14 14:34:31.205211 2018] [mpm_prefork:notice] [pid 9621] AH00169: caught SIGTERM, shutting down
[Fri Sep 14 14:34:32.378475 2018] [mpm_prefork:notice] [pid 14323] AH00163: Apache/2.4.18 (Ubuntu) OpenSSL/1.0.2g configured -- resuming normal operations
[Fri Sep 14 14:34:32.378639 2018] [core:notice] [pid 14323] AH00094: Command line: '/usr/sbin/apache2'
[Fri Sep 14 14:34:41.958284 2018] [mpm_prefork:notice] [pid 14323] AH00169: caught SIGTERM, shutting down
[Fri Sep 14 14:34:43.207179 2018] [mpm_prefork:notice] [pid 14395] AH00163: Apache/2.4.18 (Ubuntu) OpenSSL/1.0.2g configured -- resuming normal operations
[Fri Sep 14 14:34:43.207334 2018] [core:notice] [pid 14395] AH00094: Command line: '/usr/sbin/apache2'
[Fri Sep 14 14:34:57.924706 2018] [mpm_prefork:notice] [pid 14395] AH00169: caught SIGTERM, shutting down
[Fri Sep 14 14:34:59.107659 2018] [mpm_prefork:notice] [pid 14461] AH00163: Apache/2.4.18 (Ubuntu) OpenSSL/1.0.2g configured -- resuming normal operations
[Fri Sep 14 14:34:59.107740 2018] [core:notice] [pid 14461] AH00094: Command line: '/usr/sbin/apache2'
[Fri Sep 14 14:35:22.594956 2018] [mpm_prefork:notice] [pid 14461] AH00169: caught SIGTERM, shutting down
[Fri Sep 14 14:35:23.821267 2018] [mpm_prefork:notice] [pid 14554] AH00163: Apache/2.4.18 (Ubuntu) OpenSSL/1.0.2g configured -- resuming normal operations
[Fri Sep 14 14:35:23.821360 2018] [core:notice] [pid 14554] AH00094: Command line: '/usr/sbin/apache2'
[Fri Sep 14 14:43:24.365173 2018] [mpm_prefork:notice] [pid 14554] AH00169: caught SIGTERM, shutting down
[Fri Sep 14 14:43:39.910085 2018] [mpm_prefork:notice] [pid 15134] AH00163: Apache/2.4.18 (Ubuntu) OpenSSL/1.0.2g configured -- resuming normal operations
[Fri Sep 14 14:43:39.910187 2018] [core:notice] [pid 15134] AH00094: Command line: '/usr/sbin/apache2'
[Fri Sep 14 14:44:13.086601 2018] [mpm_prefork:notice] [pid 15134] AH00169: caught SIGTERM, shutting down
[Fri Sep 14 14:44:14.254920 2018] [mpm_prefork:notice] [pid 15220] AH00163: Apache/2.4.18 (Ubuntu) OpenSSL/1.0.2g configured -- resuming normal operations
[Fri Sep 14 14:44:14.255007 2018] [core:notice] [pid 15220] AH00094: Command line: '/usr/sbin/apache2'
[Fri Sep 14 14:44:41.310491 2018] [mpm_prefork:notice] [pid 15220] AH00169: caught SIGTERM, shutting down
[Fri Sep 14 14:44:42.544462 2018] [mpm_prefork:notice] [pid 15324] AH00163: Apache/2.4.18 (Ubuntu) OpenSSL/1.0.2g configured -- resuming normal operations
[Fri Sep 14 14:44:42.544558 2018] [core:notice] [pid 15324] AH00094: Command line: '/usr/sbin/apache2'
[Fri Sep 14 14:45:11.859285 2018] [mpm_prefork:notice] [pid 15324] AH00169: caught SIGTERM, shutting down
[Fri Sep 14 14:45:13.010481 2018] [mpm_prefork:notice] [pid 15430] AH00163: Apache/2.4.18 (Ubuntu) OpenSSL/1.0.2g configured -- resuming normal operations
[Fri Sep 14 14:45:13.010622 2018] [core:notice] [pid 15430] AH00094: Command line: '/usr/sbin/apache2'
[Fri Sep 14 14:45:21.782593 2018] [mpm_prefork:notice] [pid 15430] AH00169: caught SIGTERM, shutting down
[Fri Sep 14 14:45:23.084701 2018] [mpm_prefork:notice] [pid 15501] AH00163: Apache/2.4.18 (Ubuntu) OpenSSL/1.0.2g configured -- resuming normal operations
[Fri Sep 14 14:45:23.084799 2018] [core:notice] [pid 15501] AH00094: Command line: '/usr/sbin/apache2'
[Fri Sep 14 14:58:29.905524 2018] [mpm_prefork:notice] [pid 15501] AH00169: caught SIGTERM, shutting down
[Fri Sep 14 14:58:31.118739 2018] [mpm_prefork:notice] [pid 16166] AH00163: Apache/2.4.18 (Ubuntu) OpenSSL/1.0.2g configured -- resuming normal operations
[Fri Sep 14 14:58:31.118850 2018] [core:notice] [pid 16166] AH00094: Command line: '/usr/sbin/apache2'
[Fri Sep 14 15:03:46.014471 2018] [mpm_prefork:notice] [pid 16166] AH00169: caught SIGTERM, shutting down
[Fri Sep 14 15:03:47.160376 2018] [mpm_prefork:notice] [pid 16255] AH00163: Apache/2.4.18 (Ubuntu) OpenSSL/1.0.2g configured -- resuming normal operations
[Fri Sep 14 15:03:47.160531 2018] [core:notice] [pid 16255] AH00094: Command line: '/usr/sbin/apache2'
[Fri Sep 14 15:03:56.274633 2018] [mpm_prefork:notice] [pid 16255] AH00169: caught SIGTERM, shutting down
[Fri Sep 14 15:03:57.553042 2018] [mpm_prefork:notice] [pid 16324] AH00163: Apache/2.4.18 (Ubuntu) OpenSSL/1.0.2g configured -- resuming normal operations
[Fri Sep 14 15:03:57.553146 2018] [core:notice] [pid 16324] AH00094: Command line: '/usr/sbin/apache2'
[Fri Sep 14 15:19:50.725191 2018] [mpm_prefork:notice] [pid 16324] AH00169: caught SIGTERM, shutting down