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PHP Interview Questions and Answers

Test your skills through the online practice test: PHP Quiz Online Practice Test

Freshers / Beginner level questions & answers

Ques 1. What does a special set of tags <?= and ?> do in PHP?

The output is displayed directly to the browser.

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Ques 2. Would I use print "$a dollars" or "{$a} dollars" to print out the amount of dollars in this example?

In this example it wouldn't matter, since the variable is all by itself, but if you were to print something like "{$a},000,000 mln dollars", then you definitely need to use the braces.

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Ques 3. How do you define a constant?

Via define() directive, like define ("MYCONSTANT", 100);

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Ques 4. How do you pass a variable by value?

Just like in C++, put an ampersand in front of it, like $a = &$b

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Ques 5. When are you supposed to use endif to end the conditional statement?

When the original if was followed by : and then the code block without braces.

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Ques 6. Explain the ternary conditional operator in PHP?

Expression preceding the ? is evaluated, if it's true, then the expression preceding the : is executed, otherwise, the expression following : is executed.
E.g $id = isset($_GET['id']) ? $_GET['id'] : false;

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Ques 7. How do you call a constructor for a parent class?

parent::constructor($value)

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Ques 8. Why doesn't the following code print the newline properly?                 $str = 'Hello, there.nHow are you?
            print $str;
    ?>


Because inside the single quotes the n character is not interpreted as newline, just as a sequence of two characters - and n.

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Ques 9. Would you initialize your strings with single quotes or double quotes?

Since the data inside the single-quoted string is not parsed for variable substitution, it's always a better idea speed-wise to initialize a string with single quotes, unless you specifically need variable substitution.

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Ques 10. What is the difference between characters 23 and x23?

The first one is octal 23, the second is hex 23.

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Ques 11. I want to combine two variables together:

 $var1 = 'Welcome to '; $var2 = 'TechInterviews.com';
What will work faster? Code sample 1: -
$var 3 = $var1.$var2;
Or code sample 2:
$var3 = "$var1$var2";
Both examples would provide the same result - $var3 equal to "Welcome to TechInterviews.com". However, Code Sample 1 will work significantly faster. Try it out with large sets of data (or via concatenating small sets a million times or so), and you will see that concatenation works significantly faster than variable substitution.

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Ques 12. For printing out strings, there are echo, print and printf. Explain the differences.

echo is the most primitive of them, and just outputs the contents following the construct to the screen. print is also a construct (so parentheses are optional when calling it), but it returns TRUE on successful output and FALSE if it was unable to print out the string. However, you can pass multiple parameters to echo, like:

 <?php echo 'Welcome ', 'to', ' ', 'TechInterviews!'; ?>
and it will output the string "Welcome to TechInterviews!" print does not take multiple parameters. It is also generally argued that echo is faster, but usually the speed advantage is negligible, and might not be there for future versions of PHP. printf  is a function, not a construct, and allows such advantages as formatted output, but it's the slowest way to print out data out of echo, print and printf.

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Intermediate / 1 to 5 years experienced level questions & answers

Ques 13. What's the difference between include and require? -

It's how they handle failures. If the file is not found by require(), it will cause a fatal error and halt the execution of the script. If the file is not found by include(), a warning will be issued, but execution will continue.

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Ques 14. Will comparison of string "10" and integer 11 work in PHP?

Yes, internally PHP will cast everything to the integer type, so numbers 10 and 11 will be compared.

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Ques 15. Are objects passed by value or by reference?

Everything is passed by value.

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Ques 16. What's the special meaning of __sleep and __wakeup?

__sleep returns the array of all the variables than need to be saved, while __wakeup retrieves them.

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Ques 17. What's the difference between htmlentities() and htmlspecialchars()?

htmlspecialchars only takes care of <, >, single quote ', double quote " and ampersand. htmlentities translates all occurrences of character sequences that have different meaning in HTML.

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Ques 18. How do you match the character ^ at the beginning of the string?

^^

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Experienced / Expert level questions & answers

Ques 19. I am trying to assign a variable the value of 0123, but it keeps coming up with a different number, what's the problem?

PHP Interpreter treats numbers beginning with 0 as octal.

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Ques 20. How do I find out the number of parameters passed into function?

func_num_args() function returns the number of parameters passed in.

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Ques 21. If the variable $a is equal to 5 and variable $b is equal to character a, what's the value of $$b?

100, it's a reference to existing variable.

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Ques 22. What's the difference between accessing a class method via -> and via ::?

:: is allowed to access methods that can perform static operations, i.e. those, which do not require object initialization.

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Ques 23. How come the code <?php print "Contents: $arr[1]"; ?> works, but <?php print "Contents: $arr[1][2]"; ?> doesn't for two-dimensional array of mine? -

Any time you have an array with more than one dimension, complex parsing syntax is required. print "Contents: {$arr[1][2]}" would've worked.

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Ques 24. With a heredoc syntax, do I get variable substitution inside the heredoc contents?

Yes.

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Ques 25. I am writing an application in PHP that outputs a printable version of driving directions. It contains some long sentences, and I am a neat freak, and would like to make sure that no line exceeds 50 characters. How do I accomplish that with PHP?

On large strings that need to be formatted according to some length specifications, use wordwrap() or chunk_split().

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Ques 26. What's the difference between md5(), crc32() and sha1() crypto on PHP?

The major difference is the length of the hash generated. CRC32 is, evidently, 32 bits, while sha1() returns a 128 bit value, and md5() returns a 160 bit value. This is important when avoiding collisions.

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Ques 27. So if md5() generates the most secure hash, why would you ever use the less secure crc32() and sha1()?

Crypto usage in PHP is simple, but that doesn't mean it's free. First off, depending on the data that you're encrypting, you might have reasons to store a 32-bit value in the database instead of the 160-bit value to save on space. Second, the more secure the crypto is, the longer is the computation time to deliver the hash value. A high volume site might be significantly slowed down, if frequent md5() generation is required.

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