PHP interview questions and answers

  1. What does a special set of tags <?= and ?> do in PHP? - The output is displayed directly to the browser.
  2. 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.
  3. 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. Look at the similar PHP interview questions for more numeric problems.
  4. 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.
  5. How do you define a constant? - Via define() directive, like define ("MYCONSTANT", 100);
  6. How do you pass a variable by value? - Just like in C++, put an ampersand in front of it, like $a = &$b
  7. 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.
  8. 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.
  9. 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.
  10. How do I find out the number of parameters passed into function? - func_num_args() function returns the number of parameters passed in.
  11. 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.
  12. 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.
  13. Are objects passed by value or by reference? - Everything is passed by value.
  14. How do you call a constructor for a parent class? - parent::constructor($value)
  15. 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.
  16. Why doesn’t the following code print the newline properly?    <?php
                $str = ‘Hello, there.nHow are you?nThanks for visiting TechInterviews’;
                print $str;
        ?>
    Because inside the single quotes the n character is not interpreted as newline, just as a sequence of two characters - and n.
  17. 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.
  18. 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.
  19. What is the difference between characters 23 and x23? - The first one is octal 23, the second is hex 23.
  20. With a heredoc syntax, do I get variable substitution inside the heredoc contents? - Yes.
  21. 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.

  22. 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.

  23. 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().
  24. What’s the output of the ucwords function in this example?
    	$formatted = ucwords("TECHINTERVIEWS IS COLLECTION OF INTERVIEW QUESTIONS");
    	print $formatted;

    What will be printed is TECHINTERVIEWS IS COLLECTION OF INTERVIEW QUESTIONS.
    ucwords() makes every first letter of every word capital, but it does not lower-case anything else. To avoid this, and get a properly formatted string, it’s worth using strtolower() first.

  25. 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.
  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.
  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.
  28. How do you match the character ^ at the beginning of the string? - ^^
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20 Comments on PHP interview questions and answers

  1. Pushpan
    Posted 12/15/2005 at 1:29 am | Permalink

    Correction for the 11 the Question of PHP interview questions
    ————————————————————-
    # 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.

    Correction
    ———-
    $$b=$a Where as the value of $a=5;
    so the Actual answer is 5.

  2. John
    Posted 12/25/2005 at 5:24 am | Permalink

    #26 is in error

    CRC32 is 32 bits long, md5 is 128 bits, and sha1() returns a 160-bit value.

  3. Divya
    Posted 1/31/2006 at 6:50 am | Permalink

    Correction for the 11 the Question of PHP interview questions
    ————————————————————-
    # 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.

    Correction
    ———-
    $$b=$a Where as the value of $a=5;
    so the Actual answer is 5.

  4. Posted 6/30/2006 at 3:01 am | Permalink

    i like to that wht is different b/w the php4.3 to php5 ?
    how can note that string varible?

  5. phpUser
    Posted 7/22/2006 at 12:05 am | Permalink

    Correction for 6

    It asks how to pass a variable by value. The question should of course be how to pass a variable by *reference*. & does by reference, all else is by value as Q13 indicates.

  6. santraj
    Posted 10/2/2006 at 12:30 am | Permalink

    Q.N11 If the variable $a is equal to 5 and variable $b is equal to character a, what’s the value of $$b? - 5.

  7. phplearner
    Posted 12/23/2006 at 2:39 pm | Permalink

    this is really nice for PHP self-learner like me, but i like to view more and as few expert
    here made corrections, thx to them, But you should make post Q&A after recheck as learner do not confuse.

    i am looking for my first php job interview , please let me know the more interview question or free online test on php and any book or link to refer . at limelightworld AT yahooo Dot c0m

    thanks to all,

  8. Kiran
    Posted 3/15/2007 at 7:09 am | Permalink

    Correction for the 11 the Question of PHP interview questions
    ————————————————————-
    # 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.

    Correction
    ———-
    $$b=$a Where as the value of $a=5;
    so the Actual answer is 5.

  9. Daddy
    Posted 4/15/2007 at 8:15 pm | Permalink

    I just had an interview with Yahoo and they asked that what’s the fastest way to read 1 million lines from an Apache log file with PHP?

    I answered like this. I might use Unix command in PHP to split up my file to smaller sizes, then I tried to read them. I couldn’t think of any other ways. I know you could setup your Apache to split log files daily. Then, you can rotate and process them daily, but this is not the case. The log file already contains 1M lines. After interview, I made a research little bit, and I found that fseek() function allows you to read large files faster.

    Anybody knows any other ways?

  10. alok jain
    Posted 4/19/2007 at 6:58 am | Permalink

    If the variable $a is equal to 5 and variable $b is equal to character a, what’s the value of $$b?
    Ans: its would be 5.

  11. Ian
    Posted 4/26/2007 at 4:18 pm | Permalink

    13. Are objects passed by value or by reference? - Everything is passed by value.

    The correct answer is for PHP 5, it is by reference and for PHP 4 is by value.

  12. arehalliravi
    Posted 5/1/2007 at 7:59 am | Permalink

    what’s the fastest way to read 1 million lines from an Apache log file with PHP?

    can anyone answer this question pls…?

  13. demon
    Posted 5/2/2007 at 10:10 am | Permalink

    When working with VERY large files, php tends to fall over sideways and die.

    Here is a neat way to pull chunks out of a file very fast and won’t stop in mid line, but rater at end of last known line. It pulled a 30+ million line 900meg file through in ~ 24 seconds.

    NOTE:
    $buf just hold current chunk of data to work with. If you try “$buf .=” (note ‘dot’ in from of ‘=’) to append $buff, script will come to grinding crawl around 100megs of data, so work with current data then move on!


    //File to be opened
    $file = "huge.file";
    //Open file (DON'T USE a+ pointer will be wrong!)
    $fp = fopen($file, 'r');
    //Read 16meg chunks
    $read = 16777216;
    //\n Marker
    $part = 0;

    while(!feof($fp)) {
    $rbuf = fread($fp, $read);
    for($i=$read;$i > 0 || $n == chr(10);$i--) {
    $n=substr($rbuf, $i, 1);
    if($n == chr(10))break;
    //If we are at the end of the file, just grab the rest and stop loop
    elseif(feof($fp)) {
    $i = $read;
    $buf = substr($rbuf, 0, $i+1);
    break;
    }
    }
    //This is the buffer we want to do stuff with, maybe thow to a function?
    $buf = substr($rbuf, 0, $i+1);
    //Point marker back to last \n point
    $part = ftell($fp)-($read-($i+1));
    fseek($fp, $part);
    }
    fclose($fp);

  14. waylon
    Posted 5/6/2007 at 2:58 pm | Permalink

    The answer for question 4 is incorrect. Printing:
    “$a,000,000 mln dollars” will work fine since ‘,’ cannot be part of a variable name. It would not work for “$a000,000 mln dollars” though, and you would have to have the braces:
    “${a}000,000 mln dollars”. The reason for this is because the parser looks for as many tokens as it can that make up a valid variable name, so when a token such as ‘,’ is encountered, it knows that it cannot be part of the variable name.

  15. Devin
    Posted 10/2/2007 at 2:12 pm | Permalink

    Answer to Question #1 is incorrect.
    The tags tell the PHP engine that what is between these two tags is PHP code and should be interpreted. Another way of saying this is that these tags seperate the php “script” from the XHTML markup tags on a web page

  16. Trey
    Posted 10/16/2007 at 12:19 am | Permalink

    NOW A CORRECTION TO MYSELF, I USED TAGS, SHOULD HAVE USED SPECIAL CHARS

    Correction to Devin,

    Actually whatever is between the tags will be printed. It is synonymous with <?= echo ?>

    I.E. <?= ‘hello’; ?> would be the same as <?php echo ‘hello’; ?>

    Its just another, shorter way to skin a cat, but you must have the short_open_tags directive turned on in the ini.

    Cheers.

  17. nithya
    Posted 11/7/2007 at 12:18 am | Permalink

    # 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.

    Correction
    ———-
    $a=5;
    $$b=$a;
    echo $$b;

    the answer is 5

  18. debashis
    Posted 11/8/2007 at 12:46 am | Permalink

    I want to take two inputs from the user :1) year, 2)day. Then i have to find the first given day of every month of that particular year given by the user.As example (inputs are 2007 and Friday…then i have to show every first Friday of every month of the given year 2007)

  19. Scott
    Posted 2/19/2008 at 12:22 pm | Permalink

    Demon,
    With #13, that’s a pretty good technique for slurping in Apache logs one chunk at a time.

    Presumably if you’re reading in Apache logs, you want to DO something with them using iteration. You can’t easily iterate over a multi-line string… sure, you can explode() the string into an array (but doing so would double memory use!).

    I’m actually curious now how to do this so that the read creates a $rbuf_array instead of $rbuf…

  20. ashish
    Posted 12/20/2008 at 2:21 am | Permalink

    Correction to:

    How do you pass a variable by value? - Just like in C++, put an ampersand in front of it, like $a = &$b

    It is pass by reference and NOT value.

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