Double Quotes, Not Single Quotes
Posted by Phil Aaronson at 10:26 AM
I can't count the number of times I've made this same, single quote vs. double quote mistake over the years. In this latest stumble I was writing a bit of PHP code, using xpath. The line in question looked like this:
Which is trying to say, look for all the
So of course, I stared at it for too long. Then pulled a couple coworkers into my cube in the vain hope that they could save me from myself. Eventually the light bulb went off, I should have written this of course:
Damn I hate that. In PHP, like Perl before it, and like shell scripts before that, single quotes are for literal strings. Double quotes are for variable substitution, and I really wanted the variable
... $this->xml->xpath('//experiment[id=$id]') ...Which is trying to say, look for all the
<experiment> tags which have a child <id> tag whose value matches the value in $id. The only problem, when you run this you get one strange error message.Unexpected PHP error [xmlXPathEval: 1 object left on the stack] severity [E_WARNING] ...So of course, I stared at it for too long. Then pulled a couple coworkers into my cube in the vain hope that they could save me from myself. Eventually the light bulb went off, I should have written this of course:
... $this->xml->xpath("//experiment[id=$id]") ...Damn I hate that. In PHP, like Perl before it, and like shell scripts before that, single quotes are for literal strings. Double quotes are for variable substitution, and I really wanted the variable
$id to be replaced with its value. I'm getting too old.
2 Comments:
Is there any reason other than performance not to just use double quotes all the time? It may sound lazy, but those errors would be prevented.
There are times when you want the string to be explicit. I seem to go back and forth between the two as my default, and get burned either way. I can never seem to remember. I probably should just give up and use 'this '.$var.' that' all the time, and never use double quotes.
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