PHP dynamic typing behavior - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of adding string and integer

What is the output of this PHP code?

$a = "5 apples";
$b = 3;
echo $a + $b;

PHP

$a = "5 apples";
$b = 3;
echo $a + $b;

A 8

B 53

C 5 apples3

D Error

💡 Hint

PHP converts strings to numbers when using arithmetic operators.

PHP converts the string "5 apples" to the number 5 when used with +. So 5 + 3 = 8.

❓ Predict Output

intermediate

Boolean comparison with string

What is the output of this PHP code?

$x = "0";
if ($x == false) { echo "Yes"; } else { echo "No"; }

PHP

$x = "0";
if ($x == false) { echo "Yes"; } else { echo "No"; }

A Yes

B Error

C 0

D No

💡 Hint

In PHP, the string "0" is considered false in loose comparisons.

When comparing with ==, "0" is treated as false, so the condition is true and "Yes" is printed.

🔧 Debug

advanced

Why does this string concatenation fail?

Consider this PHP code:

$a = 5;
$b = " apples";
echo $a - $b;

What error or output will this produce?

PHP

$a = 5;
$b = " apples";
echo $a - $b;

A Fatal error

B 5

C 0

D Notice: A non-numeric value encountered

💡 Hint

PHP tries to convert strings to numbers in arithmetic but " apples" has no leading digits.

PHP issues a notice about non-numeric value because " apples" cannot be converted to a number, so it treats it as 0 and outputs 5 - 0 = 5 but with a notice.

❓ Predict Output

advanced

Result of loose equality with different types

What is the output of this PHP code?

$a = 0;
$b = "any string";
var_dump($a == $b);

PHP

$a = 0;
$b = "any string";
var_dump($a == $b);

A bool(false)

B bool(null)

C bool(true)

D Error

💡 Hint

When comparing number and string, PHP converts string to number. Non-numeric strings become 0.

The string "any string" converts to 0, so 0 == 0 is true, var_dump prints bool(true).

🧠 Conceptual

expert

Understanding PHP type juggling in array keys

Given this PHP code:

$arr = [];
$arr["1"] = "one";
$arr[1] = "ONE";
echo count($arr);

What is the output and why?

PHP

$arr = [];
$arr["1"] = "one";
$arr[1] = "ONE";
echo count($arr);

A 2 because string and integer keys are different

B 1 because PHP treats string "1" and integer 1 as the same key

C 0 because keys overwrite each other and array is empty

D Error due to key type mismatch

💡 Hint

PHP converts string keys that are numeric to integers in arrays.

PHP treats "1" and 1 as the same key in arrays, so the second assignment overwrites the first. The array has one element.

Practice

1. In PHP, what happens if you assign a string value to a variable and then assign an integer to the same variable later?

easy

A. PHP throws a type error because the variable type is fixed.

B. The variable keeps the original string type and ignores the integer.

C. The variable changes its type dynamically to integer.

D. The variable becomes an array containing both values.

Solution

1. Step 1: Understand PHP variable typing

   PHP variables do not have fixed types and can change type dynamically during execution.

2. Step 2: Assign different types to the same variable

   Assigning a string first and then an integer changes the variable's type to integer automatically.

3. Final Answer:

   The variable changes its type dynamically to integer. -> Option C

4. Quick Check:

   PHP variables are dynamically typed = B [OK]

Hint: Remember: PHP variables change type automatically when reassigned [OK]

Common Mistakes:

• Thinking PHP variables have fixed types like in Java
• Assuming PHP throws errors on type changes
• Believing variables keep old values as arrays

2. Which of the following is the correct way to assign a float value to a variable in PHP?

easy

A. $num = 3.14; // float assignment

B. $num = '3.14'; // string assignment

C. $num = (int)3.14; // integer cast

D. $num = float(3.14); // float function

Solution

1. Step 1: Identify correct float assignment syntax

   In PHP, assigning a float is done by directly assigning a number with decimal point without quotes.

2. Step 2: Evaluate each option

   $num = 3.14; // float assignment assigns 3.14 as a float correctly. $num = '3.14'; // string assignment assigns a string, C casts to int, D uses invalid syntax.

3. Final Answer:

   $num = 3.14; // float assignment -> Option A

4. Quick Check:

   Float assignment uses decimal number without quotes = A [OK]

Hint: Assign floats without quotes; quotes make it a string [OK]

Common Mistakes:

• Using quotes around numbers to assign floats
• Trying to cast with invalid functions like float()
• Casting floats to int unintentionally

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

$a = '5';
$b = 3;
$c = $a + $b;
echo gettype($c) . ' ' . $c;

medium

A. integer 8

B. integer 53

C. string 8

D. string 53

Solution

1. Step 1: Analyze the addition operation with mixed types

   PHP converts the string '5' to integer 5 automatically when used with + operator.

2. Step 2: Determine the result type and value

   Adding 5 + 3 gives integer 8. gettype($c) returns 'integer' and echo prints 'integer 8'.

3. Final Answer:

   integer 8 -> Option A

4. Quick Check:

   String + int converts string to int = D [OK]

Hint: PHP converts numeric strings to numbers in math operations [OK]

Common Mistakes:

• Expecting string concatenation with + operator
• Thinking result type stays string
• Confusing + with . operator for strings

4. Consider this PHP code snippet:

$x = '10 apples';
$y = 5;
$z = $x + $y;
echo $z;

What is the issue and how to fix it?

medium

A. No issue; output is 15.

B. Warning due to non-numeric string; fix by extracting number before addition.

C. Syntax error; fix by adding semicolon after $y.

D. Runtime error; fix by casting $y to string.

Solution

1. Step 1: Understand PHP's behavior with non-numeric strings in math

   PHP tries to convert '10 apples' to number; it extracts leading number 10 but may raise a warning if strict error reporting is on.

2. Step 2: Identify the fix

   To avoid warnings, explicitly extract the numeric part (e.g., using intval) before addition.

3. Final Answer:

   Warning due to non-numeric string; fix by extracting number before addition. -> Option B

4. Quick Check:

   Non-numeric strings cause warnings in math = C [OK]

Hint: Extract numbers from strings before math to avoid warnings [OK]

Common Mistakes:

• Assuming no warnings with mixed strings
• Confusing syntax errors with runtime warnings
• Casting integers to strings unnecessarily

5. You have an array of mixed values:

$values = ['10', 20, '30 apples', true, null, 5.5];
$sum = 0;
foreach ($values as $v) {
    $sum += $v;
}
echo $sum;

What will be the output and why?

hard

A. Warning and sum is 0 due to invalid types.

B. Error because null and boolean cannot be added.

C. 65 because floats are truncated in addition.

D. 66.5 because PHP converts all values to numbers in addition.

Solution

1. Step 1: Understand PHP type conversion in addition

   PHP converts strings with leading numbers to those numbers, booleans true to 1, null to 0, and floats remain floats.

2. Step 2: Calculate sum step-by-step

   '10' -> 10, 20 -> 20, '30 apples' -> 30, true -> 1, null -> 0, 5.5 -> 5.5; sum = 10+20+30+1+0+5.5 = 66.5

3. Step 3: Check calculation carefully

   Sum is 66.5; re-check addition: 10+20=30, +30=60, +1=61, +0=61, +5.5=66.5

4. Final Answer:

   66.5 because PHP converts all values to numbers in addition. -> Option D

5. Quick Check:

   PHP converts all to numbers in math = A [OK]

Hint: PHP converts all types to numbers in math operations [OK]

Common Mistakes:

• Ignoring boolean and null conversions
• Assuming floats truncate to int
• Expecting errors on mixed types