The "echo "test" line is working because echo is a bash command.
You have to write your own php script, the run it by command line like this:
$ php myscript.php
In alternative, you can run php from your command line, then directly write or paste your script.
Then press CTRL+D to run it. Remember the at the beginning and at the end.
As third option, you can write a php script, putting in the first line this code:
#!/usr/bin/php
Obviously the php executable path must match the one in your system.
This way, you can chmod +x the script, then run it directly like this:
$ ./myscript.php
The fourth option is the interactive shell:
$ php -a
Interactive shell
php > echo 5+8;
13
[$ in front of commands means a command run by user]
echobut the one of the shell ...