Curly braces in string in PHP
PHP中字符串文字中{ } （花括号）的含义是什么？
This is the complex (curly) syntax for string interpolation. From the manual:
Complex (curly) syntax
This isn't called complex because the syntax is complex, but because it allows for the use of complex expressions.
Any scalar variable, array element or object property with a string representation can be included via this 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 {$ . Some examples to make it clear:
<?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()}";
?>
Often, this syntax is unnecessary. For example, this:
$a = 'abcd';
$out = "$a $a"; // "abcd abcd";
behaves exactly the same as this:
$out = "{$a} {$a}"; // same
So the curly braces are unnecessary. But this:
$out = "$aefgh";
will, depending on your error level, either not work or produce an error because there's no variable named $aefgh , so you need to do:
$out = "${a}efgh"; // or
$out = "{$a}efgh";
As for me, curly braces serve as a substitution for concatenation, they are quicker to type and code looks cleaner. Remember to use double quotes (" ") as their content is parsed by PHP, because in single quotes (' ') you'll get the literal name of variable provided:
<?php
$a = '12345';
// This works:
echo "qwe{$a}rty"; // qwe12345rty, using braces
echo "qwe" . $a . "rty"; // qwe12345rty, concatenation used
// Does not work:
echo 'qwe{$a}rty'; // qwe{$a}rty, single quotes are not parsed
echo "qwe$arty"; // qwe, because $a became $arty, which is undefined
?>
Example:
$number = 4;
print "You have the {$number}th edition book";
//output: "You have the 4th edition book";
Without curly braces PHP would try to find a variable named $numberth , that doesn't exist!
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