Top-5 Things that might bite you For the full list see php.net/migration70 Left-to-right semantics for complicated expressions $bar['baz'] // interpreted as ($foo->$bar)['baz'] $foo->$bar['baz']() // interpreted as ($foo->$bar)['baz']() Foo::$bar['baz']() // interpreted as (Foo::$bar)['baz']() ]]> To restore the previous behaviour add explicit curly braces: {$bar['baz']} $foo->{$bar['baz']}() Foo::{$bar['baz']}() ]]> Detection: phan or unit test failures Removed support for /e (PREG_REPLACE_EVAL) modifier pres->\\1', $text); ]]> Change to: pres->{$matches[1]}; // Careful! }, $text); ]]> Detection: grep, warnings in logs or unit test failures $HTTP_RAW_POST_DATA global removed Detection: grep, warnings in logs or unit test failures session.lazy_write enabled by default Detection: Can cause out-of-band session read timing issues Invalid octal literals now produce a parse error Detecting parse errors is easy: php -l