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