I was wondering, how is the semantics of braces exactly defined inside PHP? For instance, suppose we have defined:
$a = "foo";
then what are the differences among:
echo "${a}";
echo "{$a}";
that is, are there any circumstances where the placement of the dollar sign outside the braces as opposed to within braces makes a difference or is the result always the same (with braces used to group just about anything)?
There are a lot of possibilities for braces (such as omitting them), and things get even more complicated when dealing with objects or arrays.
I prefer interpolation to concatenation, and I prefer to omit braces when not necessary. Sometimes, they are.
You cannot use object operators with ${} syntax. You must use {$...} when calling methods, or chaining operators (if you have only one operator such as to get a member, the braces may be omitted).
The ${} syntax can be used for variable variables:
$y = 'x';
$x = 'hello';
echo "${$y}"; //hello
The $$ syntax does not interpolate in a string, making ${} necessary for interpolation. You can also use strings (${'y'}) and even concatenate within a ${} block. However, variable variables can probably be considered a bad thing.
For arrays, either will work ${foo['bar']} vs. {$foo['bar']}. I prefer just $foo[bar] (for interpolation only -- outside of a string bar will be treated as a constant in that context).
${var} is deprecated and never permitted. Check the Zend Coding Style #Variable Substitution
${} altogether. I would also avoid {} except for the couple cases when it's necessary (multi-dimensional array access, method calls, object operator chaining).The brackets delimit where the variable name ends; this example should speak for itself.
$a = "hi!"
echo "$afoo"; //$afoo is undefined
echo "${a}foo"; //hi!foo
echo "{$a}foo"; //hi!foo
Also, this should spit out a warning; you should use
${'a'}
Otherwise it will attempt to assume a is a constant.
echo "{$a}foo"; should produce the same as echo "${a}foo";, the curly braces will not echo to the screen.Also you can use braces to get Char in the position $i of string $text:
$i=2;
$text="safi";
echo $text{$i}; // f
