Pressing the tab key twice when there are multiple possible completions will result in a list of these completions:
php > strp[TAB][TAB] strpbrk strpos strptime php > strp
When there is only one possible completion, pressing tab once will complete the rest on the same line:
It is also possible doing completion on things that have been defined during the interactive shell session:
php > $fooThisIsAReallyLongVariableName = 42; php > $foo[TAB]ThisIsAReallyLongVariableName