I've seen this several times in the last few days when trying to merge a MediaWiki core commit, causing the merge to be aborted.
/srv/phan/vendor/symfony/console/Terminal.php:127 [2] proc_open(): fork failed - Cannot allocate memory (Phan 1.2.6 crashed) #0 phan_error_handler() #1 proc_open() called at [/srv/phan/vendor/symfony/console/Terminal.php:127] #2 Symfony\Component\Console\Terminal::getSttyColumns() called at [/srv/phan/vendor/symfony/console/Terminal.php:70] #3 Symfony\Component\Console\Terminal::initDimensions() called at [/srv/phan/vendor/symfony/console/Terminal.php:32] #4 Symfony\Component\Console\Terminal->getWidth() called at [/srv/phan/vendor/phan/phan/src/Phan/CLI.php:1405] #5 Phan\CLI::progress() called at [/srv/phan/vendor/phan/phan/src/Phan/Analysis.php:347] #6 Phan\Analysis::analyzeFunctions() called at [/srv/phan/vendor/phan/phan/src/Phan/Phan.php:369] #7 Phan\Phan::finishAnalyzingRemainingStatements() called at [/srv/phan/vendor/phan/phan/src/Phan/Phan.php:301] #8 Phan\Phan::analyzeFileList() called at [/srv/phan/vendor/phan/phan/src/phan.php:34] #9 require_once(/srv/phan/vendor/phan/phan/src/phan.php) called at [/srv/phan/vendor/phan/phan/phan:2] More details: #1: proc_open() Args: ["stty -a | grep columns", {1:["pipe", "w"], 2:["pipe", "w"]}, null, null, null, {"suppress_errors":true}] #2: Symfony\Component\Console\Terminal::getSttyColumns() #3: Symfony\Component\Console\Terminal::initDimensions() #4: Symfony\Component\Console\Terminal->getWidth() #5: Phan\CLI::progress() Args: ["method", 0.087568759283016] #6: Phan\Analysis::analyzeFunctions() Args: [Phan\CodeBase({}), null] #7: Phan\Phan::finishAnalyzingRemainingStatements() Args: [Phan\CodeBase({}), null, [".phan/stubs/hhvm.php", ".phan/stubs/wikidiff.php", ".phan/stubs/mail.php", ".phan/stubs/password.php", ".phan/stubs/phpunit4.php", ".phan/stubs/excimer.php", "maintenance/7zip.inc", "maintenance/cleanupTable.inc", "maintenance/CodeCleanerGlobalsPass.inc", "maintenance/commandLine.inc", ... 5451 more element(s)], []] #8: Phan\Phan::analyzeFileList() Args: [Phan\CodeBase({}), Closure] #9: require_once() Args: ["/srv/phan/vendor/phan/phan/src/phan.php"] Build step 'Execute shell' marked build as failure