Global keyword behavior in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the global keyword do in PHP?

It allows a function to access variables defined outside its scope, specifically global variables, by linking the local variable to the global one.

intermediate

How do you access a global variable inside a function without using the global keyword?

You can access global variables using the $GLOBALS array, for example: $GLOBALS['varName'].

beginner

What happens if you modify a global variable inside a function without declaring it global?

The function creates a new local variable with the same name, leaving the global variable unchanged.

beginner

Example: What will this code output?<br><pre> $x = 5;<br>function test() {<br>  global $x;<br>  $x = 10;<br>}<br>test();<br>echo $x;</pre>

It will output 10 because the global keyword links the local $x to the global $x, so the change affects the global variable.

intermediate

Why is using the global keyword sometimes discouraged?

Because it can make code harder to understand and debug by creating hidden dependencies between functions and global state. It's often better to pass variables as parameters.

What does the global keyword do inside a PHP function?

A Links a local variable to a global variable of the same name

B Creates a new local variable

C Deletes a global variable

D Makes a variable private

The global keyword links the local variable to the global variable with the same name.

How can you access a global variable inside a function without using global?

A Using <code>const</code> keyword

B Using <code>static</code> keyword

C Using the <code>$GLOBALS</code> array

D You cannot access it

The $GLOBALS array holds all global variables and can be used inside functions.

What will happen if you assign a value to a global variable inside a function without declaring it global?

A The global variable changes

B The variable becomes constant

C An error occurs

D A new local variable is created

Without global, the assignment creates a new local variable.

Which of these is a downside of using the global keyword?

A It creates hidden dependencies and can make debugging harder

B It improves performance

C It makes code easier to read

D It automatically documents code

Using global can hide dependencies and make code harder to maintain.

Given $a = 3; outside a function, what will echo $a; output after this function?

function change() { global $a; $a = 7; } change();

A 3

B 7

C Error

D Nothing

The global keyword links the function's $a to the global $a, so the value changes to 7.

Explain how the global keyword works in PHP functions and why it is used.

Think about variable scope and how functions normally can't see outside variables.

Describe the alternatives to using the global keyword for accessing global variables inside functions.

Consider safer ways to share data between functions.

Practice

1. What does the global keyword do in PHP?

easy

A. It deletes a variable from the global scope.

B. It creates a new local variable inside the function.

C. It makes a variable constant and unchangeable.

D. It allows a function to access variables defined outside its scope.

Solution

1. Step 1: Understand variable scopes in PHP

   Variables defined outside functions are in the global scope and not accessible inside functions by default.

2. Step 2: Role of the global keyword

   The global keyword inside a function tells PHP to use the variable from the global scope instead of creating a new local one.

3. Final Answer:

   It allows a function to access variables defined outside its scope. -> Option D

4. Quick Check:

   global keyword = access outside variables [OK]

Hint: Global lets functions use outside variables directly [OK]

Common Mistakes:

• Thinking global creates a new variable inside the function
• Assuming global deletes variables
• Confusing global with constants

2. Which of the following is the correct way to use the global keyword inside a function to access a variable $count defined outside?

easy

A. global $count;

B. use $count;

C. global($count);

D. extern $count;

Solution

1. Step 1: Recall PHP syntax for global variables

   In PHP, to access a global variable inside a function, you write global $variableName; without parentheses.

2. Step 2: Check each option

   global $count; uses correct syntax. use $count; is invalid in PHP. global($count); uses parentheses which is incorrect. extern $count; uses 'extern' which is not a PHP keyword.

3. Final Answer:

   global $count; -> Option A

4. Quick Check:

   Correct syntax = global $var; [OK]

Hint: Use 'global $var;' without parentheses inside functions [OK]

Common Mistakes:

• Adding parentheses after global
• Using 'use' or 'extern' keywords incorrectly
• Forgetting the $ sign before variable name

3. What will be the output of the following PHP code?

$x = 5;
function test() {
  global $x;
  $x = $x + 10;
}
test();
echo $x;

medium

A. 15

B. 5

C. 10

D. Error

Solution

1. Step 1: Understand the initial value of $x

   $x is set to 5 in the global scope before the function call.

2. Step 2: Effect of the function test()

   Inside test(), global $x; makes $x inside the function refer to the global $x. Then $x is increased by 10, so $x becomes 15 globally.

3. Final Answer:

   15 -> Option A

4. Quick Check:

   global $x modified to 15 [OK]

Hint: global lets function change outside variable value [OK]

Common Mistakes:

• Thinking $x inside function is local and unchanged
• Expecting output 5 or 10 instead of 15
• Assuming error due to global keyword

4. Identify the error in this PHP code snippet:

$count = 10;
function increment() {
  $count = $count + 1;
}
increment();
echo $count;

medium

A. Variable $count is undefined.

B. Syntax error in function declaration.

C. Missing global keyword inside function to access $count.

D. Cannot echo variable outside function.

Solution

1. Step 1: Check variable scope inside function

   Inside increment(), $count is treated as a new local variable, not the global one.

2. Step 2: Missing global keyword

   To modify the global $count, the function must declare global $count; before using it.

3. Final Answer:

   Missing global keyword inside function to access $count. -> Option C

4. Quick Check:

   global needed to access outside variable [OK]

Hint: Declare global inside function to modify outside variable [OK]

Common Mistakes:

• Assuming $count inside function is global by default
• Thinking echo outside function causes error
• Confusing syntax errors with scope errors

5. Given the code below, what will be the output?

$a = 3;
$b = 4;
function sum() {
  global $a, $b;
  $b = $a + $b;
}
sum();
echo $b;

What if global $a, $b; is replaced with global $a; only?

hard

A. Output is 7 in both cases.

B. Output is 7 with global $a, $b; and 4 with global $a; only.

C. Output is 4 with global $a, $b; and 7 with global $a; only.

D. Code causes error without global $b;.

Solution

1. Step 1: When using global $a, $b;

   Both $a and $b inside the function refer to global variables. $b is updated to 3 + 4 = 7 globally.

2. Step 2: When using global $a; only

   $a inside function is global, but $b is local and uninitialized. So $b = $a + $b uses local $b (null or 0), $b becomes 3 + 0 = 3 locally, global $b remains 4. echo $b outputs global $b which is 4.

3. Final Answer:

   Output is 7 with global $a, $b; and 4 with global $a; only. -> Option B

4. Quick Check:

   global vars must be declared to modify them [OK]

Hint: Declare all globals you want to use inside function [OK]

Common Mistakes:

• Assuming global $a; also makes $b global
• Expecting output 7 without declaring global $b
• Thinking missing global causes syntax error