PHP Skills Tests

Operator precedence

PHP's operator precedence:

Associativity	Operators	Additional information
non-associative	clone new	clone and new
left	[	array()
right	**	arithmetic
right	++ -- ~ (int) (float) (string) (array) (object) (bool) @	types and increment/decrement
non-associative	instanceof	types
right	!	logical
left	* / %	arithmetic
left	+ - .	arithmetic and stirng
left	<< >>	bitwise
non-associative	< <= >= >	comparison
non-associative	== != === !== <> <=>	comparison
left	&	bitwise and references
left	^	bitwise
left	`|	bitwise
left	&&	http://php.net/manual/en/language.operators.logical.php logical]
left	||	logical
right	??	comparison
left	? :	ternary
right	= += -= *= /= .= %= &= |= ^= <<= >>= =>	assignment
left	and	logical
left	xor	logical
left	or	logical
left	,`	many uses

Multiplication, division, addition, and subtraction

echo 5 * 6 / 2 + 2 * 3; /* <<< result is 21 */
/**
 * Multiplication, division, and modulus have precedence over addition and subtraction.
 * Operations are performed left to right for operators with equal precedence.
 * So 5 * 6 = 30, 30 / 2 = 15, 2 * 3 = 6, and 15 + 6 = 21
 * Result is 21.
 */

String concatenation and string operators

echo (2).(3*(print 3)); // "323"

• This example also plays on the obscure fact that the print construct always returns the value 1.
• echo evaluates everything to the right.
• First, print 3 outputs "3".
• Then 3 is multiplied by the return value of print, i.e. 1, for a result of 3;
• 2 is concatenated to 3 for "23"
• echo outputs "23" after the "3" that has already been output, for a result of "323".

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Q: What will be the output of the following code?

$foo = 5 + "10 things";
print $foo; // 15

a.	510 things
b.	5 10 things
c.	15 things
d.	15
e.	None of the above

A: (d) 15

• One trick to this question is that there is no string concatenation operator (.) involved.
• Still one thing to keep in mind is that the dot operator has the same precedence as + and -.
• Non-empty non-numeric strings are converted to 0 when they are part of a mathematical operation.
• But a string is numeric if the numeric part comes at the beginning.
  • "10 things" = 10
  • "things 10" = 0
  • "10things" = 10
  • "10 x 4" = 10
• "10 things" is converted to 10, then added to 5.
• The final result is 15.

Associativity

TK

Data types

Initial values

Q: What is the initial value of an integer and float variable?

1. 0
2. 1
3. NULL
4. Any value

A: (C) NULL

PHP is loosely typed. An uninitialized variable doesn't have a integer or float type.

A declared class member variable that holds no data:

class Test {
  public static $someVar;
}

var_dump(Test::$someVar); // outputs NULL

Hexidecimal notation

Hex notation is indicated by a leading 0x

echo 0x500;
1280

This is hexidecimal notation. The value is calculated 0 * 1 = 0 + 0 * 16 = 0 + 5 * 256 = 1280.

Octal notation

Octal notation is indicated by a leading zero.

vardump($a = 010); // (1 * (8**1)) = 8

Q: What is the output of the following code?

echo 0500; // (5 * (8**2)) = 5 * 64  = 320

A: 320

is_numeric()

• Valid values: "200", ".25e4", "20,2"
• Invalid values: "$200"

Arrays

Joining array elements into string

Q: What function should you use to join array elements with a glue string?

1. join_str
2. implode
3. connect
4. make_array
5. None of the above

my answer: b. implode

Array comparison operators

Q: When comparing two arrays, what is the difference between == and ===?

1. == compares keys while === compares keys and values
2. === also compares the order and types of the objects
3. === compares the array references
4. They are identical
5. None of the above

A: (b) === also compares the order and types of the objects

Bitwise operators

Example	Name	Result
$a & $b	And	Bits that are set in both $a and $b are set.
`$a | $b	Or (inclusive or)	Bits that are set in either $a or $b are set.
$a ^ $b	Xor (exclusive or)	Bits that are set in $a or $b but not both are set.
~ $a	Not	Bits that are set in $a are not set, and vice versa.
$a << $b	Shift left	Shift the bits of $a $b steps to the left (each step means "multiply by two")
$a >> $b	Shift right	Shift the bits of $a $b` bits to the right (each step means "divide by two")

Bitwise operators

$a = (1<<0); /* $a = (0001) = (1*1) = 1 */
$b = (1<<1); /* $b = (0010) = (1*2) = 2 */

echo $b | $a; /* (0010 | 0001) = (0011) = ((1*2) + (1*1)) = 3 (bits set in either value are set in the result) */

Bitwise operators and precedence

Q: What will be the output of the following code?

$a = (1 << 0); /* $a = (1*1) = 1 */
$b = (1 << $a); /* $b = (1*2) = 2 */
$c = (1 << $b); /* $c = (1*4) = 4 */
echo ($c || $b) << 2 * $a | $a; 
/* ($c || $b) i.e. "4 or 2". Unless they are both false it will evaluated to true. Result is "true", or 1 */
/* multiplication > bitwise shift > bitwise or, in terms of precedence */
/* next operation, multiplication: 2 * $a, or 2 * 1 = 2 */
/* next operation, bitwise shift: 1 << 2 = 4; */
/* next operation, bitwise or: 4 | 1 = 100 | 001 = 5 */

A: 5

Bit shifting

Q: What will be the output of the following code?

$a = 0x01; // 1
$b = 0x02; // 2

echo $a === $b >> $a;

a.	0
b.	1
c.	Syntax error

A: (b) 1

• $a equals 1
• $b >> $a shifts the bit in 2 over one place, resulting in 1
• $a === $b >> $a translates to "does 1 equal 1?"
• Answer is "yes", i.e. true, i.e. 1

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Q: What will be the result of the following code?

$a = 0x01; // 1
$b = 0x02; // 2

printf("%x", ($a << $b.$b)); // 400000

• %x format for printf() means output a hexidecimal value using lowercase letters.
• $b.$b evaluates to 22.
• So, shift a bit representing 1 over 22 places, i.e. 2 to the power of 22.
• When you shift the bit over, you multiply the value by 2, so the progression is like 1, 2, 4, 8, and then 8 * 2 = 16 which in hex is 10. Then 10 * 2 in hex results in 20 which is really 16 * 2 or 2**5 or 32.
• An algorithm for translating the shift to hex would be `(2**($shift_value % 4)) . str_pad('', floor($shift_value/4), '0')`

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Q: What will be the output of the following code?

$a = (1<<0); // 0b0001, or 1
$b = (1<<1); // 0b0010, or 2 

echo ($b|$a) << 2;

a.	1
b.	2
c.	3
d.	4
e.	8
f.	12
g.	Syntax error

A: (f) 12

• ($b|$a) Or operator set the bit if it's set in either $b or $a, or 0b0010 | 0b0001 = 0b0011, or 3
• Shift 3 two places yields 0b1100 or 12
• Also, shifting a bit left in effect multiplies the value by 2 for each time the bit is shifted, so 3 * 2 * 2 = 12
• Also, shifting a bit left if effect multiplies the value by 2 to the power of the shift amount, e.g. 3 * (2**2) = 3 * 4 = 12

Magical constants

Magical constants are constants that change value depending on where in the code they are placed.

The magical constants in PHP:

• __LINE__
• __FILE__
• __DIR__
• __FUNCTION__
• __CLASS__
• __TRAIT__
• __METHOD__
• __NAMESPACE__

Language constructs

Is print() a function?

print() is a language construct.

echo ((function_exists("print"))?("y"):("n")); /* <<< output is "n" */
echo ((function_exists("printf"))?("y"):("n")); /* <<< output is "y" */
echo ((function_exists("echo"))?("y"):("n")); /* <<< output is "n" */

• Both echo and print are constructs, not functions. This makes parentheses deceiving.
• Both echo and print will output everything that follows to the right.
• echo does not return a value.
• print always returns 1, never the value to the right of it.
• Also, the string concatenation operator (.) has high precedence in PHP. But that's a red herring in the following example.

echo '1'.print(2)+3; // result: 511
/**
 * `echo` in this example prints everything that comes after it. It's necessary to evaluate everything to the right of 'echo' first.
 * `print` also outputs everything to the right, regardless of parentheses.
 * (1) Add (2) + 3 results in the value 5
 * (2) print 5 outputs 5 to stdout, and returns the value `1`.
 * (3) Concatenate '1' with the return value of 'print' results in "11"
 * (4) 'echo' the output after what's already been sent to stdout, so "11" follows the "5".
 */

Control structure alternative syntax

Q: Is the following code valid?

$a = 5;
if ($a==5):
    echo "Hello";
    echo "1";
else:
    echo "Not hello";
    echo "2";
endif;

A: Yes

Grouping operators

Q: What is empty ()?

1. A function
2. A language construct
3. A variable
4. A reference
5. None of the above

My first guess was (b) a language construct. After more considerations maybe it's more likely (e) None of the above?

They are grouping operators, or delimiting expressions. I have never seen them used without something inside. Maybe the answer is "e".

The question isn't clear about the context. I guess on their own they are meaningless.

Gettext

Q: Can PHP use Gettext?

1. yes
2. no

I don't even know what this means, or what it's getting at. There is a documentation page for gettext which says, "The gettext functions implement an NLS (Native Language Support) API which can be used to internationalize your PHP applications." Which implies that PHP can use Gettext? Support is specific to the implementation, but there are gettext functions.

No one offered an answer at this cheater's site.

Language specifications

Q: What kind of regular expression does PHP include? (multiple choice)

1. PCRE
2. Java REGEX
3. Ruby REGEX
4. None of the above

A: (a) PCRE

preg is a wrapper for the PCRE (Perl-Compatible Regular Expressions) library.

However, an answer submitted to the cheaters' site says the answer is (d) None of the above.

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Q: Is the PHP float type guaranteed to be 64 bit IEEE?

1. yes
2. no

"64 bit IEEE" translates to "double-precision floating-point format," meaning a float value occupies 64 bits (8 bytes) of memory.

• 1 bit for the sign
• 11 bits for the exponent
• 52 bits for the fraction.

From the PHP documentation:

Although it depends on the system, PHP typically uses the IEEE 754 double precision format, which will give a maximum relative error due to rounding in the order of 1.11e-16.

Which to me suggests that while you should assume that it's IEEE 754 double precision, it is not in fact guaranteed.

Built-in functions

Q: Can feof() be used with a file opened by fsockopen()?

a.	yes
b.	no

A: (a) yes

From the PHP documentation:

Parameters:

$handle

The file pointer must be valid, and must point to a file successfully opened by fopen() or fsockopen() (and not yet closed by fclose())

Security

Q: What will be the output of the following code?

$a = "Hello";
if (md5($a) === md5($a)) {
    print "True";
}
else {
    print "False";
}

a.	True
b.	False

A: (b) False

The hash that the md5() function produces for any given string will always be the same.

The question is designed to give the suggestion that perhaps md5() scrambles a string.