Curly braces are used to delimit variables in double quoted strings in PHP. While very useful in many situations, this can cause confusion when trying to print a combination of curly braces and variables. For example, suppose you are trying to use PHP to generate the following Javascript code:

// Desired Javascript code
var obj = {foo:1, bar:2};

The incorrect PHP code might look something like this:

// PHP code to create Javascript code (incorrect)
$data = "foo:1, bar:2";
echo "<script>var obj = {$data};</script>";

This fails because the curly braces are used to interpret the $data variable, and thus the curly braces are not outputted. The actual output of the above code would be:

// Incorrect Javascript code created by PHP
var obj = foo:1, bar:2;

This is obviously incorrect and will throw a Javascript error.

Now, the funny thing is you can’t escape the curly braces in the PHP code. However, the curly braces are interpreted as a delimiter only if followed by a variable (in other words, a dollar sign). You can take advantage of this to generate the correct output in several ways:

// PHP code to create correct Javascript code
$data = "foo:1, bar:2";
echo "var obj = {{$data}};";
echo "var obj = { $data };";
echo "var obj = { $data};";

When you are actually trying to use the curly braces to define a variable, they can be placed either before or after the dollar sign, so ${foo} and {$foo} both work. The former is called simple syntax and will greedily take as many tokens as possible to form a valid variable name. The latter is called complex syntax and can be used with complex expressions, which can prove quite useful. Here are some examples:

// Value of the index 'bar' in the array $foo
echo "{$foo['bar']}";

// Value of the property 'bar' in the object $foo
echo "{$foo->bar}";

// Value of the property named $bar in the object $foo
echo "{$foo->$bar}";

// Value of the var named $name
echo "{${$name}}";

// Value of the var named by the return value of the function getName()
echo "{${getName()}}";

// Var named by the return value of $object->getName()
echo "{${$object->getName()}}";