PHP：如何用另一个数组中的一个替换数组中的每个字符

Question

I want to replace each character from an array with one in another array:

<?php
$str="a b c d e f g h i j ";

$pattern=array();
$pattern[0]="a";
$pattern[1]="b";
$pattern[2]="c";
$pattern[3]="d";
$pattern[4]="e";
$pattern[5]="f";
$pattern[6]="g";
$pattern[7]="h";
$pattern[8]="i";
$pattern[9]="j";

$replacement=array();
$replacement[0]="6a";
$replacement[1]="6e";
$replacement[2]="6i";
$replacement[3]="6o";
$replacement[4]="6u";
$replacement[5]="5a";
$replacement[6]="5e";
$replacement[7]="5i";
$replacement[8]="5o";
$replacement[9]="5u";

echo str_replace($pattern,$replacement,$str);?>

RESULT:

    6a 66u 65o 6o 6u 5a 5e 55o 5o 5u
    a   b   c   d  e  f  g  h   i  j

the 'b' 'c' 'h' kind conflict with the others

Answer 1

When using str_replace with arrays() as $search and $replace parameters, the function will evaluate every items in the $search array, for EACH characters of the $subject string. This is important to understand as every "loop" for a given character evaluates the content of the position of the $subject that it is working on at that point.

This results in the function being able to change that character multiple times in one pass (that character from the $subject, against all the items in the $search array.)

A simple example would be the following code/output:

$str = "a b c";
$pattern = array("a","b","a");
$replacement = array("a1","b1","c1");
echo str_replace($pattern, $replacement, $str); // Output : c11 b1 c

The function executes the "search" against ALL items in the search array, for each character in $str. Here is the first "loop" for the first character in $str ("a") :

First is finds "a", which gets replaced with "a1", then it looks for "b", which is not found. Then it looks for "a" again, and replaces the aforementioned replacement ("a"=>"a1") with the mapping "a"=>"c1" which leads to "c11".

Then goes on to the next character in $str.

YOUR EXAMPLE

When the second character in your $str ("b") is searched-against, it gets replaced with "6e" (this is the new state of $str) then following the "loop" that NEW "e" is found in the $search array, and replaced with "6u". At that point, you have "6a 66u". You can extrapolate the rest.

The reason why the first "a"=>"6a" is exact, is that the NEW state of $str after first iteration of the search "loop", that is "6a", will not match any of the other items in the search array.

As per php documentation ( php.net str_replace() ) :

Caution: Replacement order gotcha. Because str_replace() replaces left to right, it might replace a previously inserted value when doing multiple replacements.

This behavior has led to very interesting/unexpected results and was not trivial to debug.

Some answers already point to solutions, I wanted to chime-in to provide details about the experienced behavior of php.

Hope this helps, let me know! Cheers