I can make out the difference between
echo "{$var1}someString" // here the variable is $var1
echo "$var1someString" // here the variable is $var1someString
The question is why to use {}? It works only with {}. It does not work with (). What is so special about { }?
you expained it yourself. thats simply the syntax php uses for this - nothing more. to quote the documentation:
Complex (curly) syntax
Simply write the expression the same way as it would appear outside the string, and then wrap it in { and }. Since { can not be escaped, this syntax will only be recognised when the $ immediately follows the {. Use {\$ to get a literal {$
The curly braces {} are used in that way to identify variables within strings:
echo "{$var1}someString"
If you look at:
echo "$var1someString"
PHP can't possibly determine that you wanted to echo $var1, it's going to take all of it as the variable name.
You could concatenate your variables instead:
echo $var1 . "someString"
It doesn't work for () simply because the PHP designers choose {}.
According to the documentation of string, the curly part is called complex syntax. It basically allows you to use complex expressions inside string.
Example from the documentation:
<?php
// Show all errors
error_reporting(E_ALL);
$great = 'fantastic';
// Won't work, outputs: This is { fantastic}
echo "This is { $great}";
// Works, outputs: This is fantastic
echo "This is {$great}";
echo "This is ${great}";
// Works
echo "This square is {$square->width}00 centimeters broad.";
// Works, quoted keys only work using the curly brace syntax
echo "This works: {$arr['key']}";
// Works
echo "This works: {$arr[4][3]}";
// This is wrong for the same reason as $foo[bar] is wrong outside a string.
// In other words, it will still work, but only because PHP first looks for a
// constant named foo; an error of level E_NOTICE (undefined constant) will be
// thrown.
echo "This is wrong: {$arr[foo][3]}";
// Works. When using multi-dimensional arrays, always use braces around arrays
// when inside of strings
echo "This works: {$arr['foo'][3]}";
// Works.
echo "This works: " . $arr['foo'][3];
echo "This works too: {$obj->values[3]->name}";
echo "This is the value of the var named $name: {${$name}}";
echo "This is the value of the var named by the return value of getName(): {${getName()}}";
echo "This is the value of the var named by the return value of \$object->getName(): {${$object->getName()}}";
// Won't work, outputs: This is the return value of getName(): {getName()}
echo "This is the return value of getName(): {getName()}";
?>
Here {} define that variable within these are not a simple string so php pic up that variable value instead of assuming that as a simple string.