PHP为什么在str_replace中的第四个参数不起作用？

Question

I'm a beginner in PHP and I can't understand why this does not work.

<?php
$count = 1;
$body = 'Biceps, Triceps, Quadriceps, Chest, Calves, Forearms are great to train!';
$body = str_replace('Chest', 'Quadriceps', $body);
$body = str_replace('Calves', 'Triceps', $body);
$body = str_replace('Forearms', 'Biceps', $body);
$body = str_replace('Biceps', 'Forearms', $body, $count);
$body = str_replace('Triceps', 'Calves', $body, $count);
$body = str_replace('Quadriceps', 'Chest', $body, $count);
echo $body;
?>

For the latter three replacements, although the fourth parameter, which supposedly defines the number of replacements is set to 1, they still perform the replacement twice and I end up with this output:

'Forearms, Calves, Chest, Chest, Calves, Forearms are great to train!'

instead of:

'Forearms, Calves, Chest, Quadriceps, Triceps, Biceps are great to train!'

...why?!

Answer 1

What the manual say is:

If passed, this will be set to the number of replacements performed.

Additionally, the function signature features the & symbol to indicate that the function writes into the variable you pass to it:

mixed str_replace ( mixed $search , mixed $replace , mixed $subject [, int &$count ] )

In short, it's a report, not a setting.

Answer 2

Each replace happens before the next is performed.

• In the third replacement, you're changing "Forearms" to "Biceps."
• In the fourth replacement, you're changing it back when you change
  "Biceps" to "Forearms."

Similar problem with 2nd and 4th replacements. The "1" you're passing in to str_replace doesn't do anything.

Consider using preg_replace if you only want the first instance to be replaced.

Answer 3

The fourth parameter (from the manual )...

count

If passed, this will be set to the number of replacements performed.

So not the amount to do, but the amount done.

Just as an alternative, try...

$body1 = strtr($body, 
    ['Chest'=>'Quadriceps','Calves' => 'Triceps', 
        'Forearms' => 'Biceps', 'Biceps' => 'Forearms',
        'Triceps' => 'Calves', 'Quadriceps' => 'Chest'
    ] );

Which outputs

Forearms, Calves, Chest, Quadriceps, Triceps, Biceps are great to train!