Local Variable

$technology = "PHP";
displayTechnology();

function displayTechnology() {
      $technology = "HTML"; // this is the local variable, not the one defined outside of this function
      echo $technology;
}

echo $technology;

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Global Variable

global $technology; // define global variable first and then assign the value after.
$technology = "PHP";
displayTechnology();

function displayTechnology {
      global $technology; // to access global variable, need to define it again.
      $technology = "HTML"; // change the value of the global variable.
}

echo $technology;

Another (recommended) way of accessing global variable

(See documentation of Superglobals And Predefined Variables)

global $ranking; // define global variable first and then assign the value after.
$ranking = "Medium";
displayRanking();

function displayRanking {
      $GLOBALS['ranking'] = "Top"; // another way of accessing global variable (recommended).
}

echo $ranking;

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Static Variable

$counter(); // prints 1
$counter(); // prints 2
$counter(); // prints 3

function counter {
      static $count = 0;
      $count = $count + 1;
      echo $count;
}

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Variable Variables (a variable can hold another variable)

$clientNames = "ClientA, ClientB, ClientC";
$clients = "clientNames";
echo $$clients;

$clientNames = "ClientA, ClientB, ClientC";
$clients = "clientNames";
$allClients = "clients";
echo $$$allClients;

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isset() function

if (isset($address)) {
      echo $address;
} else {
      echo "address variable is not defined.";
}

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