哪些字符可以使用简单的语法直接跟随解析的PHP变量？

Question

For instance, the following will not print "hello john_doe" as desired, since $first_ is not defined.

$first = 'john';
$last = 'doe';

echo "hello $first_$last";

Maybe there are none?

Answer 1

Any character that cannot be part of a variable name can be used there - if the character can be part of the variable name, PHP cannot determine whether it's part of it or not...

According to the manual:

Variable names follow the same rules as other labels in PHP. A valid variable name starts with a letter or underscore, followed by any number of letters, numbers, or underscores. As a regular expression, it would be expressed thus: '[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*'

So, to answer your question: any character other than a letter, a number, or an underscore can immediately follow a parsed PHP variable using simple syntax...
(Note that special characters like ƒ, ©, √, etc... fall under that \x7f-\xff UTF-8 range in the regexp and are valid characters).

With some exceptions - e.g., you can't use:

• [ because it's considered an access to an array key.
• {$ because it's considered as an attempt to use a variable (e.g., {$var}).
• -> followed by a character that can be part of a variable, because it's considered an access to a property (e.g., ->prop).