PHP variables

Local Variables

Variables defined within a function can only be used within that function.

function test() {
    $localVar = "Local variable";
    echo $localVar;
}
test();
echo $localVar; // Will cause an error

Output

Global Variables

Variables defined outside of functions cannot be used directly inside functions. However, using the global keyword makes them accessible inside functions.

$globalVar = "Global variable";
function testGlobal() {
    global $globalVar;
    echo $globalVar;
}
testGlobal();

You can also use the $GLOBALS array to access global variables.

function testGlobalArray() {
    echo $GLOBALS['globalVar'];
}
testGlobalArray();

Static Variables

Static variables are variables that retain their value even when defined inside a function. Normally, local variables are re-initialized each time the function is called and are discarded after execution. By using the static keyword, the variable value is retained across function calls.

function counter() {
    static $count = 0; // Initialize as static variable
    $count++;          // Increment on each call
    echo $count;
}

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

Output

Why use static?

• To retain state
  Useful when you want to retain results from previous calls. In this example, the count is maintained across calls.
• Difference from global variables
  You can retain state within the function without using global variables, avoiding unnecessary scope pollution.
• To avoid recalculating every time
  Ideal for functions that are called multiple times but require only one-time calculation.

Checking Variable Types

PHP provides several functions to check the type of a variable.

var_dump($variable); // Output detailed variable info
is_int($variable);   // Check if variable is an integer (returns true/false)
is_string($variable);// Check if variable is a string (returns true/false)

var_dump()

A handy function that shows both the “type” and “value” of a variable. For example, it displays length and content for strings, and number of elements for arrays.

is_int() / is_string() etc.

is_XXX() functions return true / false based on type. Others include is_bool(), is_float(), is_array(), is_object(), etc. Use as needed.

Checking Variable Existence

isset($variable); // Check if variable is set
empty($variable); // Check if variable is empty

isset()

Checks whether a variable is defined and not null. Returns false if undefined or null, otherwise true.

empty()

Checks if a variable is “empty” (e.g., 0, “”, “0”, [], null, etc.).
Returns true if any of the following are true:

• Value is an empty string ""
• Value is 0 or “0”
• Value is null
• Array is empty []
• Variable is undefined
• Boolean value is false

If none apply, returns false.

Difference between isset() and empty()

• isset() checks if a variable exists and is not null.
• empty() checks if a value is empty (0, “”, [], null, false, etc.).

Deleting Variables

unset($variable); // Delete the variable

unset()

Deletes the specified variable, making it no longer exist. Accessing it afterward may cause an error or warning. Re-defining with the same name will treat it as a new variable.