Default parameter values in PHP - Step-by-Step Execution

Concept Flow - Default parameter values

Function call

↓

Check if argument given

↓

Use arg

↓

Execute function body

↓

Return result

When a function is called, PHP checks if an argument is given. If yes, it uses that argument; if not, it uses the default value set in the function.

Execution Sample

PHP

<?php
function greet($name = "Friend") {
    echo "Hello, $name!";
}
greet();
greet("Alice");
?>

This code defines a function with a default parameter and calls it with and without an argument.

Execution Table

Step	Function Call	Parameter $name	Action	Output
1	greet()	Not given	Use default 'Friend'	Hello, Friend!
2	greet("Alice")	"Alice"	Use given argument	Hello, Alice!
3	End	-	No more calls	-

💡 No more function calls, execution ends.

Variable Tracker

Variable	Start	After Call 1	After Call 2	Final
$name	undefined	"Friend"	"Alice"	undefined

Key Moments - 2 Insights

Why does the function print 'Hello, Friend!' when called without arguments?

Because the parameter $name has a default value 'Friend', so when no argument is given (see execution_table step 1), PHP uses that default.

What happens if you call the function with an argument?

PHP uses the given argument instead of the default (see execution_table step 2), so it prints 'Hello, Alice!'.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of $name during the first function call?

A "Friend"

B "Alice"

C undefined

D null

💡 Hint

Check the 'Parameter $name' column in step 1 of the execution_table.

At which step does the function use the argument provided by the caller?

A Step 1

B Step 2

C Step 3

D None

💡 Hint

Look at the 'Action' column in the execution_table for step 2.

If you call greet("Bob"), what will be the output?

A Hello, Friend!

B Hello, Alice!

C Hello, Bob!

D Error

💡 Hint

Based on the pattern in the execution_table, the argument replaces the default.

Concept Snapshot

function greet($name = "Friend") {
  // If no argument, $name = "Friend"
  echo "Hello, $name!";
}

- Default values used when argument missing
- Argument overrides default
- Helps avoid errors from missing parameters

Full Transcript

This visual execution shows how PHP functions handle default parameter values. When the function greet is called without an argument, PHP uses the default value 'Friend' for the parameter $name, printing 'Hello, Friend!'. When called with an argument like 'Alice', PHP uses that value instead, printing 'Hello, Alice!'. The execution table tracks each call, showing parameter values and outputs. The variable tracker shows how $name changes during calls. This helps beginners understand how default parameters provide fallback values and how arguments override them.

Practice

1. What is the purpose of default parameter values in PHP functions?

easy

A. To force the user to always pass an argument

B. To make the function run faster

C. To provide a value if no argument is passed when calling the function

D. To declare a variable inside the function

Solution

1. Step 1: Understand default parameters

   Default parameters give a preset value to a function argument if no value is provided during the call.

2. Step 2: Identify the purpose

   This helps avoid errors and makes the function flexible by not requiring all arguments every time.

3. Final Answer:

   To provide a value if no argument is passed when calling the function -> Option C

4. Quick Check:

   Default parameter = fallback value [OK]

Hint: Default values fill in missing arguments automatically [OK]

Common Mistakes:

• Thinking default values speed up the function
• Believing default values force argument passing
• Confusing default parameters with variable declarations

2. Which of the following is the correct syntax to define a function with a default parameter value in PHP?

easy

A. function greet($name = "Guest") { echo "Hello, $name!"; }

B. function greet($name) = "Guest" { echo "Hello, $name!"; }

C. function greet($name: "Guest") { echo "Hello, $name!"; }

D. function greet($name) { $name = "Guest"; echo "Hello, $name!"; }

Solution

1. Step 1: Review PHP default parameter syntax

   In PHP, default values are assigned using the equals sign inside the function parameter list.

2. Step 2: Check each option

   function greet($name = "Guest") { echo "Hello, $name!"; } uses correct syntax: $name = "Guest" inside parentheses. Syntaxes using = after the closing parenthesis or colon (:) are invalid. function greet($name) { $name = "Guest"; echo "Hello, $name!"; } assigns default inside the function body, not as a parameter default.

3. Final Answer:

   function greet($name = "Guest") { echo "Hello, $name!"; } -> Option A

4. Quick Check:

   Default parameter = equals sign in parentheses [OK]

Hint: Use = inside parentheses to set default values [OK]

Common Mistakes:

• Using colon or other symbols instead of =
• Assigning default inside function body instead of parameter list
• Placing default value outside parentheses

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

function multiply($a, $b = 2) {
  return $a * $b;
}
echo multiply(5);

medium

A. 7

B. 5

C. Error: Missing argument

D. 10

Solution

1. Step 1: Analyze function call with one argument

   The function multiply expects two parameters, but the second has a default value of 2.

2. Step 2: Calculate the result

   Calling multiply(5) uses $a=5 and default $b=2, so 5 * 2 = 10.

3. Final Answer:

   10 -> Option D

4. Quick Check:

   5 * default 2 = 10 [OK]

Hint: If argument missing, default value is used [OK]

Common Mistakes:

• Expecting error for missing second argument
• Using wrong default value
• Confusing output with sum instead of product

4. Identify the error in this PHP function definition:

function greet($name = "Guest", $age) {
  echo "Hello $name, age $age";
}

medium

A. Function cannot have default parameters

B. Default parameter must be last or followed only by default parameters

C. Missing semicolon after echo statement

D. Variable names cannot have default values

Solution

1. Step 1: Check parameter order rules

   In PHP, parameters with default values must come after parameters without defaults.

2. Step 2: Identify the error

   Here, $name has a default but $age does not, and $age comes after $name, which is invalid syntax.

3. Final Answer:

   Default parameter must be last or followed only by default parameters -> Option B

4. Quick Check:

   Default parameters must be at the end [OK]

Hint: Place all default parameters after non-default ones [OK]

Common Mistakes:

• Putting default parameters before required ones
• Ignoring PHP syntax rules for parameter order
• Thinking semicolon or variable name causes error

5. Consider this PHP function:

function createUser($name, $role = "user", $active = true) {
  return "$name is a $role and active status is " . ($active ? "yes" : "no");
}

// Call the function
$result = createUser("Alice", active: false);
echo $result;

What will be the output and why?

hard

A. Alice is a user and active status is no

B. Alice is a user and active status is yes

C. Syntax error due to incorrect argument order

D. Error: Named arguments not supported

Solution

1. Step 1: Understand named arguments and defaults

   PHP 8+ supports named arguments, so calling createUser("Alice", active: false) sets $active to false and uses default $role = "user".

2. Step 2: Evaluate function output

   $name = "Alice", $role = "user", $active = false, so output is "Alice is a user and active status is no".

3. Final Answer:

   Alice is a user and active status is no -> Option A

4. Quick Check:

   Named argument sets active=false, role defaults [OK]

Hint: Named arguments can skip defaults, set specific params [OK]

Common Mistakes:

• Assuming named arguments are unsupported in PHP
• Expecting syntax error due to argument order
• Confusing default parameter usage with named arguments