PHP using double quotes or single quotes in a function difference

Hey I have some simple php/html:

  if( $_POST["name"] || $_POST["age"] )
  {
     echo "Welcome ". $_POST['name']. "
";
     echo "You are ". $_POST['age']. " years old.";
     exit();
  }
?>





  

  Name: 
  Age: 
  
  

I don't understand why on the second line we use double quotes i.e. $_POST["name"] but in the echo statement we use $_POST['name'], could someone tell me why these are different? Completely new to PHP...

Answer

It doesn't really matter what quotes you choose. There are several discussions about performance. Indeed double quotes are a tiny bit slower. But the difference is minimal even at 100,000+ iterations.

The only differences are the concenation and escaping:

Concenation

Double quotes:

$b = 1;
echo "Test ".$b; // outputs Test 1
echo "Test $b"; // also outputs Test 1

Single quotes:

$b = 2;
echo 'Test '.$b; // outputs Test 2

echo 'Test $b'; // outputs Test $b

Escaping

Double quotes:

echo "\t";
// outputs actual tab letter

Single quotes

echo '\t';
// outputs \t as plain text.