You can always manually write array assignments explicitly, without references, like this: Therefore: STOP using the old, mythological "&$val" iteration method! It's almost always BAD! With worse performance, and risks of bugs and quirks as is demonstrated in the manual. So PHP makes a FULL COPY of the ENTIRE array and ALL VALUES before it starts iterating. If we just give them references to the original array's values, and they assign some new value to their reference, they would destroy the original array which they aren't allowed to touch!". But it also sees that you want to look at all VALUES by reference (&$val), so PHP says "Uh oh, this is dangerous. The array itself wasn't passed by reference to the function, so PHP knows that it isn't allowed to modify the outside array. If you try to MODIFY $val, THEN it will allocate a NEW zval in memory and store $val there instead (but it still won't modify the original array, so you can rest assured).Īlright, so what's the second version doing? The beloved "iterate values by reference"? It's called "copy-on-write" and means that PHP doesn't make any copies unless you try to MODIFY the value. Does $val make a COPY of the value? That's what MANY people think. And the value at the current offset (a PHP "zval") is assigned to a variable called $val. Next, it uses that copied iteration offset to loop through all key/value pairs of the array (ie 0th key, 1st key, 2nd key, etc.). The array itself wasn't passed by reference to the function, so PHP knows that it isn't allowed to modify the outside array, so it therefore makes a copy of the array's internal iteration offset state (that's just a simple number which says which item you are currently at during things like foreach()), which costs almost no performance or memory at all since it's just a small number. The array function argument itself isn't passed by reference, so the function knows it isn't allowed to modify the original at all. This function takes an array as argument ($arr). Lots of people think the answer is two() because it uses "reference to value, which it doesn't have to copy each value when it loops". WARNING: Looping through "values by reference" for "extra performance" is an old myth. Getting Started Introduction A simple tutorial Language Reference Basic syntax Types Variables Constants Expressions Operators Control Structures Functions Classes and Objects Namespaces Errors Exceptions Generators Attributes References Explained Predefined Variables Predefined Exceptions Predefined Interfaces and Classes Context options and parameters Supported Protocols and Wrappers Security Introduction General considerations Installed as CGI binary Installed as an Apache module Session Security Filesystem Security Database Security Error Reporting User Submitted Data Hiding PHP Keeping Current Features HTTP authentication with PHP Cookies Sessions Dealing with XForms Handling file uploads Using remote files Connection handling Persistent Database Connections Command line usage Garbage Collection DTrace Dynamic Tracing Function Reference Affecting PHP's Behaviour Audio Formats Manipulation Authentication Services Command Line Specific Extensions Compression and Archive Extensions Cryptography Extensions Database Extensions Date and Time Related Extensions File System Related Extensions Human Language and Character Encoding Support Image Processing and Generation Mail Related Extensions Mathematical Extensions Non-Text MIME Output Process Control Extensions Other Basic Extensions Other Services Search Engine Extensions Server Specific Extensions Session Extensions Text Processing Variable and Type Related Extensions Web Services Windows Only Extensions XML Manipulation GUI Extensions Keyboard Shortcuts ? This help j Next menu item k Previous menu item g p Previous man page g n Next man page G Scroll to bottom g g Scroll to top g h Goto homepage g s Goto search
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