Operator precedence and evaluation in PHP - Step-by-Step Execution

Concept Flow - Operator precedence and evaluation

Start Expression

↓

Identify Operators

↓

Check Precedence

↓

Evaluate Highest Precedence Operator

↓

Replace Result in Expression

↓

More Operators?

Yes→Check Precedence

No↓

Final Result

The expression is evaluated by first identifying operators, then applying them in order of precedence until the final result is obtained.

Execution Sample

PHP

<?php
$result = 3 + 4 * 2;
echo $result;
?>

Calculates 3 plus 4 times 2, showing how multiplication happens before addition.

Execution Table

Step	Expression	Operator Evaluated	Operation	Resulting Expression	Output
1	3 + 4 * 2	*	4 * 2 = 8	3 + 8
2	3 + 8	+	3 + 8 = 11	11
3	11	None	No more operators	11	11

💡 No more operators left, final result is 11

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
result	undefined	undefined	11	11

Key Moments - 2 Insights

Why does multiplication happen before addition in the expression?

Because multiplication has higher precedence than addition, as shown in step 1 of the execution_table where 4 * 2 is evaluated before adding 3.

What if we want addition to happen first?

We can use parentheses to change the order, for example (3 + 4) * 2, which would evaluate addition first.

Visual Quiz - 3 Questions

Look at the execution_table, what is the expression after step 1?

A 11

B 7 * 2

C 3 + 8

D 3 + 4 * 2

💡 Hint

Check the 'Resulting Expression' column in step 1 of the execution_table.

At which step is the final result calculated?

A Step 1

B Step 2

C Step 3

D No final result

💡 Hint

Look at the 'Operation' and 'Resulting Expression' columns to see when the final calculation happens.

If the expression was changed to (3 + 4) * 2, what would be the result after the first operation?

A 7 * 2

B 3 + 8

C 11

D 3 + 4 * 2

💡 Hint

Parentheses change precedence, so addition happens first, resulting in 7 * 2.

Concept Snapshot

Operator precedence determines the order in which parts of an expression are calculated.
Multiplication (*) has higher precedence than addition (+).
Expressions are evaluated from highest precedence to lowest.
Parentheses () override precedence to force evaluation order.
In PHP, this affects how expressions like 3 + 4 * 2 are calculated.

Full Transcript

This visual execution shows how PHP evaluates an expression with multiple operators by following operator precedence rules. First, it identifies the operators in the expression 3 + 4 * 2. Since multiplication has higher precedence than addition, it calculates 4 * 2 first, resulting in 8. Then it adds 3 to 8, giving the final result 11. The variable 'result' is updated only after the full expression is evaluated. Parentheses can change this order by forcing addition first. Understanding this helps avoid mistakes in calculations.

Practice

1. In PHP, which operator has the highest precedence among these: +, *, -, /?

easy

A. All have the same precedence

B. Addition (+)

C. Subtraction (-)

D. Multiplication (*) and division (/)

Solution

1. Step 1: Recall operator precedence rules

   Multiplication (*) and division (/) have higher precedence than addition (+) and subtraction (-).

2. Step 2: Compare given operators

   Among +, -, *, /, the * and / operators are evaluated first.

3. Final Answer:

   Multiplication (*) and division (/) -> Option D

4. Quick Check:

   Higher precedence = * and / [OK]

Hint: Remember: multiply/divide before add/subtract [OK]

Common Mistakes:

• Thinking addition has higher precedence
• Assuming all operators have equal precedence
• Confusing subtraction with division precedence

2. Which of the following PHP expressions is syntactically correct and respects operator precedence?

easy

A. $result = 5 + 3 * 2;

B. $result = 5 + * 3 2;

C. $result = 5 + (3 * 2;

D. $result = 5 + 3 *;

Solution

1. Step 1: Check syntax of each expression

   $result = 5 + 3 * 2; is a valid PHP expression with correct operator placement. Options B, C, and D have syntax errors like misplaced operators or missing parentheses.

2. Step 2: Confirm operator precedence in $result = 5 + 3 * 2;

   Multiplication (*) happens before addition (+), so expression is valid and clear.

3. Final Answer:

   $result = 5 + 3 * 2; -> Option A

4. Quick Check:

   Valid syntax and precedence = $result = 5 + 3 * 2; [OK]

Hint: Check for misplaced operators and balanced parentheses [OK]

Common Mistakes:

• Leaving out parentheses
• Placing operators without operands
• Unbalanced parentheses

3. What is the output of the following PHP code?

$a = 4 + 2 * 3 - 1;

medium

A. 11

B. 17

C. 9

D. 15

Solution

1. Step 1: Apply operator precedence

   Multiplication (*) happens before addition (+) and subtraction (-). So calculate 2 * 3 = 6 first.

2. Step 2: Calculate the full expression

   Expression becomes 4 + 6 - 1 = 9. Then 4 + 6 = 10, 10 - 1 = 9.

3. Final Answer:

   9 -> Option A

4. Quick Check:

   Multiplication first, then add/subtract = 9 [OK]

Hint: Multiply before adding or subtracting [OK]

Common Mistakes:

• Adding before multiplying
• Subtracting before multiplying
• Ignoring operator precedence

4. Find the error in this PHP expression:

$result = 10 - 4 / 2 *;

medium

A. Division operator used incorrectly

B. Missing operand after the last operator

C. Subtraction operator has wrong precedence

D. Expression is correct

Solution

1. Step 1: Analyze the expression syntax

   The expression ends with an operator '*', but no operand follows it. This is a syntax error.

2. Step 2: Confirm the error type

   Operators must have operands on both sides. Missing operand after '*' causes error.

3. Final Answer:

   Missing operand after the last operator -> Option B

4. Quick Check:

   Operator must have operands both sides [OK]

Hint: Check if every operator has two operands [OK]

Common Mistakes:

• Ignoring missing operand errors
• Thinking operator precedence causes error
• Assuming expression is valid

5. Given the PHP expression:

$value = 8 + 2 * (5 - 3) / 2 - 1;

What is the correct value of $value after evaluation?

hard

A. 9

B. 10

C. 8

D. 11

Solution

1. Step 1: Evaluate parentheses first

   Calculate (5 - 3) = 2.

2. Step 2: Apply multiplication and division next

   Calculate 2 * 2 = 4, then 4 / 2 = 2.

3. Step 3: Perform addition and subtraction left to right

   Calculate 8 + 2 = 10, then 10 - 1 = 9.

4. Final Answer:

   8 -> Option C

5. Quick Check:

   Parentheses, then multiply/divide, then add/subtract = 8 [OK]

Hint: Calculate parentheses first, then multiply/divide, then add/subtract [OK]

Common Mistakes:

• Ignoring parentheses
• Doing addition before multiplication
• Dividing before multiplying inside parentheses