Parent keyword behavior in PHP - Step-by-Step Execution

Concept Flow - Parent keyword behavior

Define Parent Class with method

↓

Define Child Class extends Parent

↓

Override method in Child

↓

Call parent::method() inside Child method

↓

Execute Child method

↓

Parent method runs, then Child continues

Shows how a child class calls a method from its parent class using the parent keyword inside an overridden method.

Execution Sample

PHP

<?php
class ParentClass {
  function greet() {
    echo "Hello from Parent\n";
  }
}

class ChildClass extends ParentClass {
  function greet() {
    parent::greet();
    echo "Hello from Child\n";
  }
}

$obj = new ChildClass();
$obj->greet();
?>

This code shows a child class calling its parent's greet method before adding its own message.

Execution Table

Step	Action	Evaluation	Output
1	Create ChildClass object	Object created
2	Call greet() on ChildClass object	ChildClass::greet() runs
3	Inside ChildClass::greet(), call parent::greet()	ParentClass::greet() runs	Hello from Parent
4	Continue ChildClass::greet() after parent call	Echo child message	Hello from Child
5	Method greet() ends	Return to caller

💡 Method greet() completes after printing both parent and child messages

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
$obj	null	ChildClass object	ChildClass object	ChildClass object	ChildClass object

Key Moments - 2 Insights

Why do we use parent::greet() inside the child method?

Using parent::greet() calls the original method from the parent class, allowing the child to add behavior without replacing the parent's code entirely, as shown in step 3 of the execution_table.

What happens if we omit parent::greet() in the child method?

If omitted, only the child's message prints because the parent's greet method is not called, so the output would miss "Hello from Parent", as seen by comparing step 3 and 4.

Visual Quiz - 3 Questions

Look at the execution_table, what is printed at step 3?

A "Hello from Child"

B "Hello from Parent"

C Nothing is printed

D An error occurs

💡 Hint

Check the Output column at step 3 in the execution_table

At which step does the child class add its own message?

A Step 4

B Step 3

C Step 2

D Step 5

💡 Hint

Look at the Action and Output columns in the execution_table for when "Hello from Child" is printed

If we remove parent::greet() call, what changes in variable_tracker?

A $obj becomes null

B $obj changes to ParentClass object

C No change in $obj variable values

D $obj is undefined

💡 Hint

Variable $obj remains the same object regardless of method calls, see variable_tracker

Concept Snapshot

Parent keyword in PHP lets a child class call a method from its parent class.
Use parent::methodName() inside an overridden method.
This allows extending behavior without replacing it.
Without parent::, only child method runs.
Useful for code reuse and adding extra steps.

Full Transcript

This example shows how a child class in PHP can call a method from its parent class using the parent keyword. First, a ParentClass defines a greet method that prints a message. Then, ChildClass extends ParentClass and overrides greet. Inside ChildClass's greet, it calls parent::greet() to run the parent's method, then prints its own message. When we create a ChildClass object and call greet, the output shows the parent's message followed by the child's message. The execution table traces each step: creating the object, calling the method, calling the parent method, printing the child's message, and ending the method. The variable tracker shows the object stays the same throughout. Key moments clarify why parent:: is used and what happens if omitted. The quiz tests understanding of output at each step and variable state. This helps beginners see how inheritance and method overriding work with parent keyword in PHP.

Practice

1. What does the parent keyword do in PHP classes?

easy

A. It renames the parent class.

B. It creates a new instance of the parent class.

C. It deletes the parent class methods from the child class.

D. It allows a child class to access methods or properties from its parent class.

Solution

1. Step 1: Understand the role of parent keyword

   The parent keyword in PHP is used inside a child class to refer to its parent class.

2. Step 2: Identify what parent allows

   It allows the child class to call or extend methods or properties defined in the parent class.

3. Final Answer:

   It allows a child class to access methods or properties from its parent class. -> Option D

4. Quick Check:

   parent accesses parent methods/properties [OK]

Hint: Remember: parent calls parent class features [OK]

Common Mistakes:

• Thinking parent creates new objects
• Confusing parent with $this
• Believing parent deletes methods

2. Which of the following is the correct syntax to call a parent class method named show inside a child class method?

easy

A. parent.show();

B. parent->show();

C. parent::show();

D. parent:show();

Solution

1. Step 1: Recall PHP syntax for calling parent methods

   In PHP, to call a parent class method inside a child, use the scope resolution operator :: with parent.

2. Step 2: Match the correct syntax

   The correct syntax is parent::methodName();, so for method show, it is parent::show();.

3. Final Answer:

   parent::show(); -> Option C

4. Quick Check:

   Use :: with parent to call methods [OK]

Hint: Use double colon (::) to call parent methods [OK]

Common Mistakes:

• Using arrow (->) instead of double colon (::)
• Using dot (.) operator which is for strings
• Using single colon (:) which is invalid syntax

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

class A {
  public function greet() {
    return "Hello from A";
  }
}

class B extends A {
  public function greet() {
    return parent::greet() . " and B";
  }
}

$b = new B();
echo $b->greet();

medium

A. Hello from A

B. Hello from A and B

C. Hello from B

D. Error: Cannot call parent method

Solution

1. Step 1: Understand method overriding and parent call

   Class B overrides greet() but calls parent::greet() which returns "Hello from A".

2. Step 2: Concatenate parent result with " and B"

   The child method returns the parent's string plus " and B", so the full string is "Hello from A and B".

3. Final Answer:

   Hello from A and B -> Option B

4. Quick Check:

   parent::greet() + " and B" = "Hello from A and B" [OK]

Hint: parent::method() returns parent's output, can concatenate [OK]

Common Mistakes:

• Ignoring parent:: call and expecting only child's string
• Thinking it causes an error
• Confusing output with only parent's or only child's string

4. Identify the error in this PHP code:

class ParentClass {
  public function sayHi() {
    echo "Hi from parent";
  }
}

class ChildClass extends ParentClass {
  public function sayHi() {
    parent.sayHi();
    echo " and child";
  }
}

$obj = new ChildClass();
$obj->sayHi();

medium

A. Using dot (.) instead of scope resolution (::) to call parent method

B. Missing parentheses after method name

C. Child class cannot override parent methods

D. Parent class method is private

Solution

1. Step 1: Check how parent method is called

   The code uses parent.sayHi(); which is invalid syntax in PHP.

2. Step 2: Correct syntax uses scope resolution operator

   It should be parent::sayHi(); to call the parent method inside child.

3. Final Answer:

   Using dot (.) instead of scope resolution (::) to call parent method -> Option A

4. Quick Check:

   Use parent::method(), not parent.method() [OK]

Hint: Use ::, not . to call parent methods [OK]

Common Mistakes:

• Using dot (.) operator for method calls
• Forgetting parentheses after method name
• Assuming private methods can be called by child

5. Given these classes, what will be the output?

class Base {
  protected function message() {
    return "Base message";
  }
}

class Derived extends Base {
  public function message() {
    return parent::message() . " extended";
  }
}

class MoreDerived extends Derived {
  public function message() {
    return parent::message() . " and more";
  }
}

$obj = new MoreDerived();
echo $obj->message();

hard

A. Base message extended and more

B. Base message and more

C. Error: Cannot access protected method

D. Base message extended

Solution

1. Step 1: Trace method calls through inheritance

   MoreDerived::message() calls parent::message() which is Derived::message().

2. Step 2: Derived::message() calls Base::message()

   Derived calls parent::message() which is Base::message() returning "Base message" and appends " extended".

3. Step 3: Combine all returned strings

   So Derived returns "Base message extended" and MoreDerived appends " and more" resulting in "Base message extended and more".

4. Final Answer:

   Base message extended and more -> Option A

5. Quick Check:

   Chained parent calls build full string [OK]

Hint: Each parent:: call adds to string stepwise [OK]

Common Mistakes:

• Expecting error due to protected method (allowed in child)
• Ignoring chained parent calls
• Assuming only base message prints