What are braces {} in an SQL string for?

Question

What are braces for {} in the following SQL string used in PHP for Mysql?

$sql = "INSERT INTO {$table} ({$fields}) VALUES ({$placeholders})"

Answer 1

Keys {} followed by a dollar $ indicate that the complex syntax string interpolation is being used, it serves to invoke methods of an object. It also serves to display the value of a variable (somewhat exaggerated or necessary in a very particular case) this is only valid within double quotes. Static members NAY can be invoked inside double quotes in this case it is mandatory to concatenate the strings

<?php
class Pessoa {
    const FOO = 'algum valor';

    public $nome = 'mario';
    public $idade = '20';
    public static $planeta = 'terra';

    public function getNumeroAleatorioDaSorte(){
        return 6;
    }
}

$pessoa = new Pessoa();

//exemplos validos:
echo "{$pessoa->getNumeroAleatorioDaSorte()} <br>";
echo "{$pessoa->nome} <br>";
echo "$pessoa->nome <br>";

//exemplos invalidos
echo "$pessoa->getNumeroAleatorioDaSorte()";
//notice: Undefined property Pessoa::$getNumeroAleatorioDaSorte

echo "Pessoa::$planeta";//notice: Undefined variable: planeta e imprime Planeta::
echo "{Pessoa::$planeta}";//notice: Undefined variable: planeta e imprime {Pessoa::}

echo "Pessoa::FOO";//imprime Pessoa::FOO
echo "{Pessoa::FOO}";//imprime {Pessoa::FOO}

Example

Answer 2

Using double-quoted keys allows calling methods to generate a value to be concatenated into the string.

It is valid to remember that it will work only with double quotes.

In this your example you could quietly remove the keys ({}).

Answer 3

What are braces {} in the following SQL string used in PHP for Mysql?

In this case for nothing. PHP will interpret the keys and return you something like this:

$tabela = 'user';
$fields = 'nome,email';
$placeholders = "'Jorge','[email protected]'";
$sql = "INSERT INTO {$table} ({$fields}) VALUES ({$placeholders})"

echo $sql; // INSERT INTO user (nome,email) VALUES ('Jorge','[email protected]')

Not going too deep, it might be more interesting to use Prepared statments and have darlings specific per table instead of a string generic susceptible to SQL Injection.

---

When PHP finds a $ in a string in double quotes " or Heredocs, it will try to interpret the value of the variable.

Because it is a string, we can have characters that blend with the name of our variables or we want to concatenate in the string a more complex structure, such as the so-called method of an object or a array for example.

That’s where the keys come in (or braces) {}. Next to the dollar sign $ they act as an extra delimiter, enabling the use of simple or complex expressions.

Simple Expressions

When we wish to display a position of array or an object property for example, or even a simple variable where the continuation of text influences the variable that is called.

Examples:

<?php
$cerveja = 'Heineken';

// funciona, "'" é um caractere inválido para nome de variáveis
echo "O sabor das '$cerveja's é ótimo"; 

// não funciona, 's' é um caractere válido para nome de variáveis
// e o php procurará a variável $cervejas
echo "Ele bebeu algumas $cervejas";

echo "Ele bebeu algumas ${cerveja}s";   // funciona
echo "Ele bebeu algumas {$cerveja}s";   // funciona

$fruits = ['morango' => 'vermelho', 'banana' => 'amarelo'];

// Funciona
echo "A banana é {$fruits['banana']}.";

// Funciona, mas o PHP procura por uma constante chamada 'banana' antes,
// gerando um Notice no seu código.
echo "A banana é {$fruits[banana]}."

$obj = new stdClass();
$obj->property = 'molezinha';

echo "Strings em php é {$obj->property}";

Some of these expressions also work if keys are omitted, for example $obj->property, but the use of the keys gives us more control to display exactly what we want.

Complicated Expressions

Key syntax also works with complex expressions, such as multi-dimensional vectors, called methods, functions, or variable variables.

Examples:

<?php

echo "Isto funciona: {$arr['foo'][3]}";
echo "Isto funciona também {$obj->values[3]->name}";    
echo "Este é o valor da variável chamada \$name: {${$name}}";    
echo "Este é o valor da variável usando o valor retornado da getName(): {${getName()}}";    
echo "Este é o valor da variável 
usando o valor retornado da \$object->getName(): {${$object>getName()}}";

$Bar = "a";
$Foo = "Bar";
$World = "Foo";
$Hello = "World";
$a = "Hello";

echo $a;           //Rertorna Hello
echo "$$a";        //Rertorna World
echo "${$$a}";     //Rertorna Foo     - Mais de um nível de variável variável
echo "${$$$a}";    //Rertorna Bar     - precisamos das chaves
echo "${$$$$a}";   //Rertorna a

---

Personally, I always try to include the keys when I will directly display complex structures in PHP.

When I use simple variables, case the string does not collide with valid variable names, I simply omit keys {}.

Examples inspired by PHP documentation