Evaluate a string as PHP code
eval ( string $code ) : mixed
Valid PHP code to be evaluated.
The code must not be wrapped in opening and closing PHP tags, i.e. 'echo "Hi!";' must be passed instead of '<?php echo "Hi!"; ?>'. It is still possible to leave and re-enter PHP mode though using the appropriate PHP tags, e.g. 'echo "In PHP mode!"; ?>In HTML mode!<?php echo "Back in PHP mode!";'. Apart from that the passed code must be valid PHP. This includes that all statements must be properly terminated using a semicolon. 'echo "Hi!"' for example will cause a parse error, whereas 'echo "Hi!";' will work. A return statement will immediately terminate the evaluation of the code. The code will be executed in the scope of the code calling eval(). Thus any variables defined or changed in the eval() call will remain visible after it terminates.
eval() returns NULL unless return is called in the evaluated code, in which case the value passed to return is returned. As of PHP 7, if there is a parse error in the evaluated code, eval() throws a ParseError exception. Before PHP 7, in this case eval() returned FALSE and execution of the following code continued normally. It is not possible to catch a parse error in eval() using set_error_handler().
$string1 and $string2
Hi! and Hello!