<?php
$string = <<<EOF
\$databases['default']['default']['init_commands']['sql_mode'] = "SET sql_mode = 'REAL_AS_FLOAT,PIPES_AS_CONCAT,ANSI_QUOTES,IGNORE_SPACE,STRICT_TRANS_TABLES,STRICT_ALL_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO'";
EOF;

file_put_contents('sites/default/settings.php', $string . "\n", FILE_APPEND);