Why use a backslash \ before a variable???? help!!!

Paul_help_

Hi good people.

I need some help with this small piece of code below:

$mode = “+”;
$processNumbers = create_function( ‘$a, $b’, “return \$a $mode \$b;” );
echo $processNumbers( 2, 3 );

The outcome of this code displays '5'.

But the thing i don't understand is, why there are backslashes before the variables:

\$a and \$b ??????

why not just $a and $b ?????

Paul.

Weedpacket

See the description for [man]create_function[/man] in the manual. It's the same reason you'd escape dollar signs in other double-quoted strings.

Paul_help_

hey. can i just ask.. when php people refer to 'haystack' and 'needle',,, do they mean the whole code (haystack) and a snippet of that code (needle)??

bradgrafelman

Some context might help, but it's almost assuredly a reference to the idiom/cliché "a needle in a haystack." If in the context of functions like [man]strstr/man, "haystack" is the collection of data to be searched, "needle" is thing for which you're searching.

Paul_help_

could you give me an example, indicating the haystack and the needle, if possible?

Paul_help_

also.... back to my original question. when you say 'escape dollar signs',, does this mean to make it so you can print or echo the literal variable name as a literal sting output??

eg:

$variable = 5;

echo $variable

output:

$variable,,,,,,,,,, not '5'

bradgrafelman

Paul help!;11033077 wrote:
could you give me an example, indicating the haystack and the needle, if possible?

$haystack = "Hello, world!";
$needle = ",";

if(strpos($haystack, $needle) !== FALSE)
    echo "\$haystack contains \$needle!";
else
    echo "\$haystack doesn't contain \$needle!";

Paul help!;11033079 wrote:
also.... back to my original question. when you say 'escape dollar signs',, does this mean to make it so you can print or echo the literal variable name as a literal sting output??

Sort of the other way around; you can include what looks like an actual variable name inside of a string. See my above (dual-purpose) example code; the echo messages will contain a literal dollar sign followed by the word 'needle', not a comma (i.e. the contents of the variable $needle).

Paul_help_

ok. this is what i don't understand...............

in the above example you just gave me: \$haystack and \$needle variables are escaped... like you said, they will only echo as \$haystack and \$needle and not there containing value.

But in this example:

$mode = “+”;
$processNumbers = create_function( ‘$a, $b’, “return \$a $mode \$b;” );
echo $processNumbers( 2, 3 );

........... “return \$a $mode \$b;” .......... \$a and \$b are escaped but still seem to contain there values.

in the book i am reading, it does not give a reason for this.

how on earth can $mode add to escaped variables together?

bradgrafelman

In that code, the string contains literal dollar signs followed by an "a" and a "b". This is because [man]create_function/man takes the contents of that string and executes it as if it were PHP code.

EDIT:

how on earth can $mode add to escaped variables together?

It doesn't; the code being executed is:

return $a + $b;

If you didn't escape $a and $b inside the double quote delimited string, you'd get undefined variable errors since $a and $b don't exist at that point.

Paul_help_

ok. so if it were in single quotes, there would be no need to escape the variables?

bradgrafelman

Right... except you'd have to use concatenation since $mode is meant to be interpolated rather than appear literally in the string.

Paul_help_

could you show me what you mean by that by showing me the same code but in single quotes?

bradgrafelman

This:

"return \$a $mode \$b;"

is equivalent to this:

'return $a ' . $mode . ' $b;';