unexpected behavior with PHP extract() function using EXTR_PREFIX_IF_EXISTS

Question

I was trying to figure out the behavior of EXTR_PREFIX_IF_EXISTS flag in extract() function. But when I run my code, it adds more than the desired number of variables(two in this case) to current symbol table as indicated by the line:variables added. The output indicates three variables were added instead of two and variable 'key3' exists. Now I can't figure out what's causing this behavior or whether I misunderstood this concept. I'm new to PHP. Any help will be really appreciated.

my code:

$key1 = 'old';
$key2 = 'old';
$my_array = array (
    'key1'  => 'new value1',
    'key2'  => 'new value2',
    'key3'  => 'new value3'
);

$num = extract ($my_array, EXTR_PREFIX_IF_EXISTS, "prefixed");
echo "variables added:$num<br />"; //number of variables imported to symbol table
echo isset($key1) ? 'TRUE': 'FALSE';
echo "<br />";// true
echo isset($prefixed_key3) ? 'TRUE': 'FALSE'; 
echo "<br />";
echo isset($key3) ? 'TRUE': 'FALSE';// should output false

output:

variables added:3
TRUE
FALSE
TRUE

Try print_r(get_defined_vars()); which will show you what variables are defined (and so which ones you create) — Nigel Ren
– Nigel Ren, Commented Sep 19, 2018 at 6:48
I did as Nigel said and it works!. now it displays only two variables were added. thanks Nigel. — S.Fernando
– S.Fernando, Commented Sep 19, 2018 at 7:04
That's really weird because adding a print_r shouldn't have changed the output... the behaviour you were seeing was definitely incorrect, you should only see that behaviour with the EXTR_PREFIX_SAME flag. This demo on rextester demonstrates the correct behaviour of each flag. — Nick
– Nick, Commented Sep 19, 2018 at 7:08
@Nick - it shouldn't (and I think your answer is correct, so worth undeleting), the suggestion was more a debugging aid than an answer. — Nigel Ren
– Nigel Ren, Commented Sep 19, 2018 at 7:11
@NigelRen actually my answer was wrong, I had the flag behaviour mixed up with EXTR_PREFIX_SAME. One wonders if OP had changed the flag but then not uploaded the new code??? — Nick
– Nick, Commented Sep 19, 2018 at 7:13

Elementary · Accepted Answer · 2018-09-19 08:08:06Z

1

 EXTR_PREFIX_IF_EXISTS

means prefix existing variable name when name collisions occur before extracting the variable to the scope and create a new variable if no collision occurs.In your case you didn't define $Key3 anywhere before the extract function so the actual behavior is completely normal...

In your code extract proceed actually this way:

loop through your array and check if the variable already exists. If it exists, according to your flag, create a new variable with the name prefix+_+the existing name . In your case you already defined $key1 and $key2 before the use of extract so 2 variables are created with prefixed name but as already said the variable $key3 is undefined before extract so it creates one more variable without prefix .Thus make the number of variables added correct(3 in your case).

edited Sep 19, 2018 at 8:08

answered Sep 19, 2018 at 7:52

Elementary

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