Spaceship operator in PHP - Step-by-Step Execution

Concept Flow - Spaceship operator

Evaluate left expression

↓

Evaluate right expression

↓

Compare left and right

↓

Return -1 if left < right

↓

Return 0 if left == right

↓

Return 1 if left > right

↓

End

The spaceship operator compares two values and returns -1, 0, or 1 depending on whether the left value is less than, equal to, or greater than the right value.

Execution Sample

PHP

<?php
$result = 5 <=> 3;
echo $result;
?>

This code compares 5 and 3 using the spaceship operator and prints the result.

Execution Table

Step	Left Expression	Right Expression	Comparison	Result
1	5	3	5 > 3	1
2	2	2	2 == 2	0
3	1	4	1 < 4	-1
4	-	-	-	End of comparisons

💡 All comparisons done, operator returns -1, 0, or 1 based on comparison

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
result	undefined	1	0	-1	varies per comparison

Key Moments - 3 Insights

Why does the spaceship operator return 1 when left is greater than right?

Because the operator is designed to return 1 when the left value is greater, as shown in execution_table row 1 where 5 > 3 returns 1.

What happens if both values are equal?

The operator returns 0, indicating equality, as shown in execution_table row 2 where 2 == 2 returns 0.

Can the spaceship operator be used with strings?

Yes, it compares strings lexicographically, returning -1, 0, or 1 just like with numbers.

Visual Quiz - 3 Questions

Look at the execution table, what is the result when left is 1 and right is 4?

A -1

B 1

C 0

D Undefined

💡 Hint

Check execution_table row 3 where left=1 and right=4

At which step does the spaceship operator return 0?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at execution_table row 2 where left and right are equal

If the left expression is less than the right, what does the spaceship operator return?

A 0

B -1

C 1

D It throws an error

💡 Hint

See execution_table row 3 for left < right case

Concept Snapshot

Spaceship operator (<=>) compares two values.
Returns -1 if left < right.
Returns 0 if left == right.
Returns 1 if left > right.
Useful for sorting and comparisons.
Works with numbers and strings.

Full Transcript

The spaceship operator in PHP compares two values and returns -1, 0, or 1 depending on whether the left value is less than, equal to, or greater than the right value. For example, 5 <=> 3 returns 1 because 5 is greater than 3. When both values are equal, like 2 <=> 2, it returns 0. If the left value is less, like 1 <=> 4, it returns -1. This operator is useful for sorting and comparing values easily.

Practice

1. What does the PHP spaceship operator <=> return when comparing two equal values?

easy

A. 1

B. -1

C. 0

D. true

Solution

1. Step 1: Understand the spaceship operator behavior

   The spaceship operator returns -1 if the left value is less, 0 if equal, and 1 if greater.

2. Step 2: Apply to equal values

   When both values are equal, the operator returns 0.

3. Final Answer:

   0 -> Option C

4. Quick Check:

   Equal values return 0 [OK]

Hint: Equal values always return 0 with <=> operator [OK]

Common Mistakes:

• Thinking it returns true or false
• Confusing with comparison operators like == or ===
• Assuming it returns boolean instead of integer

2. Which of the following is the correct syntax to compare two variables $a and $b using the spaceship operator in PHP?

easy

A. $a <=> $b;

B. $a => $b;

C. $a <==> $b;

D. $a <-> $b;

Solution

1. Step 1: Recall the spaceship operator syntax

   The spaceship operator is written as <=> between two values.

2. Step 2: Check each option

   $a <=> $b; uses <=> correctly. Others have invalid or incorrect symbols.

3. Final Answer:

   $a <=> $b; -> Option A

4. Quick Check:

   Correct operator is <=> [OK]

Hint: Spaceship operator is exactly <=> with no extra symbols [OK]

Common Mistakes:

• Using => which is for arrays
• Adding extra equals signs
• Using arrows like <-> which is invalid

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

$x = 5;
$y = 10;
echo $x <=> $y;

medium

A. -1

B. 1

C. 0

D. 10

Solution

1. Step 1: Compare values 5 and 10 using spaceship operator

   Since 5 is less than 10, the operator returns -1.

2. Step 2: Output the result

   The echo statement prints -1.

3. Final Answer:

   -1 -> Option A

4. Quick Check:

   5 < 10 returns -1 [OK]

Hint: Left less than right returns -1 with <=> [OK]

Common Mistakes:

• Confusing output with boolean true or false
• Expecting the right value to print
• Thinking it returns 1 when left is smaller

4. Identify the error in the following PHP code using the spaceship operator:

$a = 7;
$b = 3;
if ($a <=> $b) {
    echo 'a is greater';
}

medium

A. Variables $a and $b are not defined

B. Spaceship operator cannot be used in if condition

C. Missing semicolon after echo statement

D. The condition always evaluates to true except when values are equal

Solution

1. Step 1: Understand spaceship operator in if condition

   The operator returns -1, 0, or 1. In PHP, 0 is false, others are true.

2. Step 2: Analyze the condition with $a=7 and $b=3

   Since 7 > 3, it returns 1, so condition is true and echo runs.

3. Final Answer:

   The condition always evaluates to true except when values are equal -> Option D

4. Quick Check:

   Non-zero spaceship result is true in if [OK]

Hint: Spaceship returns 0 (false) only if values equal [OK]

Common Mistakes:

• Thinking spaceship operator returns boolean
• Expecting it to directly compare like > or <
• Ignoring that 0 is false and others true in if

5. Given an array $numbers = [3, 1, 4, 1, 5], which code snippet correctly sorts it in ascending order using the spaceship operator in a custom comparison function?

hard

A. usort($numbers, fn($a, $b) => $a == $b);

B. usort($numbers, fn($a, $b) => $a <=> $b);

C. sort($numbers, fn($a, $b) => $a <=> $b);

D. usort($numbers, fn($a, $b) => $b <=> $a);

Solution

1. Step 1: Understand sorting with spaceship operator

   To sort ascending, compare $a and $b with $a <=> $b.

2. Step 2: Check correct function usage

   usort accepts a comparison function; sort does not take a callback.

3. Final Answer:

   usort($numbers, fn($a, $b) => $a <=> $b); -> Option B

4. Quick Check:

   Ascending sort uses $a <=> $b with usort [OK]

Hint: Use usort with fn($a,$b) => $a <=> $b for ascending sort [OK]

Common Mistakes:

• Using sort() with a callback function
• Reversing $a and $b for ascending order
• Using equality check instead of comparison