Type coercion in operations in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is type coercion in PHP?

Type coercion is when PHP automatically converts one data type to another during operations, like turning a string into a number when needed.

beginner

What happens when you add a string containing a number to an integer in PHP?

$result = '5' + 3;

PHP converts the string '5' to the number 5, then adds 3, so $result becomes 8.

intermediate

How does PHP treat a string that starts with numbers but has letters after when used in arithmetic?

$result = '10abc' + 5;

PHP reads the starting number 10 and ignores the letters, so it adds 10 + 5 = 15.

beginner

What is the result of adding a boolean to an integer in PHP?

$result = true + 2;

PHP converts true to 1, so the result is 1 + 2 = 3.

intermediate

What happens when you add a non-numeric string to a number in PHP?

$result = 'hello' + 5;

PHP converts the non-numeric string to 0, so the result is 0 + 5 = 5.

What does PHP do when adding '7' + 3?

A Keeps '7' as string and ignores 3

B Concatenates '7' and 3 to '73'

C Throws an error

D Converts '7' to number 7 and adds to 3

PHP automatically converts the string '7' to the number 7 and then adds 3, resulting in 10.

What is the result of '12abc' + 8 in PHP?

A 20

B 12abc8

C Error

D 8

PHP reads the numeric part '12' and adds 8, resulting in 20.

How does PHP treat true when used in arithmetic?

A As 1

B As 0

C As true

D Throws an error

PHP converts true to 1 in arithmetic operations.

What happens when you add 'hello' + 5 in PHP?

A Error

B hello5

C 5

D hello

Non-numeric strings convert to 0, so 0 + 5 = 5.

Which of these is NOT a type PHP coerces automatically?

A Boolean to number

B Array to number

C String to number

D Integer to float

PHP does not automatically convert arrays to numbers in arithmetic operations.

Explain how PHP handles type coercion when adding a string and a number.

Think about how PHP reads the string before adding.

Describe what happens when a boolean is used in an arithmetic operation in PHP.

Booleans act like small numbers in math.

Practice

1. In PHP, what happens when you add a string containing a number to an integer, like '5' + 3?

easy

A. PHP throws a type error because you can't add string and integer.

B. The result is the string '53' by concatenating '5' and '3'.

C. The string '5' is converted to the number 5, then added to 3, resulting in 8.

D. The operation returns null because of incompatible types.

Solution

1. Step 1: Understand PHP type coercion in addition

   PHP automatically converts strings containing numbers to integers or floats when used in arithmetic operations.

2. Step 2: Apply addition after conversion

   The string '5' becomes the integer 5, then 5 + 3 equals 8.

3. Final Answer:

   8 -> Option C

4. Quick Check:

   '5' + 3 = 8 [OK]

Hint: Strings with numbers convert to numbers in math operations [OK]

Common Mistakes:

• Thinking PHP concatenates strings with + operator
• Expecting a type error on mixed types
• Assuming the result stays a string

2. Which of the following PHP expressions correctly adds a boolean true to an integer 2 and returns 3?

easy

A. (int)'true' + 2

B. true + 2

C. true . 2

D. 'true' + 2

Solution

1. Step 1: Understand boolean to integer conversion

   In PHP, boolean true converts to integer 1 in arithmetic operations.

2. Step 2: Evaluate each option

   (int)'true' + 2: (int)'true' is 0 + 2 = 2 (wrong). 'true' + 2: 'true' converts to 0 + 2 = 2 (wrong). true . 2: concatenation '12' (wrong). true + 2: 1 + 2 = 3 (correct).

3. Final Answer:

   true + 2 -> Option B

4. Quick Check:

   true + 2 = 3 [OK]

Hint: Boolean true acts as 1 in math operations [OK]

Common Mistakes:

• Confusing string 'true' with boolean true
• Using concatenation instead of addition
• Not knowing boolean converts to 1 or 0

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

$a = '10 apples';
$b = 5;
echo $a + $b;

medium

A. 105

B. 10 apples5

C. Error: Unsupported operand types

D. 15

Solution

1. Step 1: Understand string to number conversion in arithmetic

   PHP converts the string '10 apples' to the number 10 by reading digits from the start until non-digit characters.

2. Step 2: Perform addition

   10 + 5 equals 15, so echo outputs 15.

3. Final Answer:

   15 -> Option D

4. Quick Check:

   '10 apples' + 5 = 15 [OK]

Hint: Strings starting with numbers convert to those numbers in math [OK]

Common Mistakes:

• Expecting string concatenation with +
• Thinking it causes an error
• Assuming the whole string converts to zero

4. Consider this PHP code snippet:

$x = '5 cats';
$y = 3;
$result = $x - $y;
echo $result;

Why might this code produce an unexpected result?

medium

A. PHP converts '5 cats' to 5, so 5 - 3 = 2, which might be unexpected if you expect string behavior.

B. Because subtraction is not defined for strings, it causes a fatal error.

C. The variable $result will be the string '5 cats3' due to concatenation.

D. PHP treats both as strings and outputs '5 cats3'.

Solution

1. Step 1: Analyze subtraction with string and integer

   PHP converts the string '5 cats' to the number 5 for arithmetic operations.

2. Step 2: Calculate the subtraction

   5 - 3 equals 2, so $result is 2, which may surprise if expecting string concatenation or error.

3. Final Answer:

   PHP converts '5 cats' to 5, so result is 2 -> Option A

4. Quick Check:

   '5 cats' - 3 = 2 [OK]

Hint: Subtraction forces string to number conversion [OK]

Common Mistakes:

• Expecting string concatenation with - operator
• Thinking it causes an error
• Assuming result is a string

5. You want to compare two variables in PHP: $a = '0' (string) and $b = false (boolean). Which comparison returns true when using type coercion, and which returns false when checking type strictly?

Choose the correct pair:

hard

A. $a == $b is true; $a === $b is false

B. $a == $b is false; $a === $b is true

C. $a == $b is true; $a === $b is true

D. $a == $b is false; $a === $b is false

Solution

1. Step 1: Understand loose (==) vs strict (===) comparison

   Loose comparison converts types before comparing; strict compares value and type exactly.

2. Step 2: Evaluate comparisons

   Loose: '0' converts to 0, false converts to 0, so 0 == 0 is true. Strict: '0' is string, false is boolean, types differ, so false.

3. Final Answer:

   $a == $b is true; $a === $b is false -> Option A

4. Quick Check:

   '0' == false is true; '0' === false is false [OK]

Hint: Use === to check type and value, == allows type coercion [OK]

Common Mistakes:

• Assuming == checks type strictly
• Thinking '0' and false are always different
• Confusing loose and strict equality