preg_match如何返回匹配？

According to PHP manual "If matches is provided, then it is filled with the results of search. $matches[0] will contain the text that matched the full pattern, $matches[1] will have the text that matched the first captured parenthesized subpattern, and so on."

How can I return a value from a string with only knowing the first few characters?

The string is dynamic and will always change whats inside, but the first four character will always be the same.

For example how could I return "Car" from this string "TmpsCar". The string will always have "Tmps" followed by something else.

From what I understand I can return using something like this

preg_match('/(Tmps+)/', $fieldName, $matches);

echo($matches[1]);

Should return "Car".

Your regex is flawed. Use this:

preg_match('/^Tmps(.+)$/', $fieldName, $matches);
echo($matches[1]);

$matches = []; // Initialize the matches array first
if (preg_match('/^Tmps(.+)/', $fieldName, $matches)) {
    // if the regex matched the input string, echo the first captured group
    echo($matches[1]);
}

Note that this task could easily be accomplished without regex at all (with better performance): See startsWith() and endsWith() functions in PHP.

An alternative way is using the explode function

$fieldName= "TmpsCar";
$matches = explode("Tmps", $fieldName);
if(isset($matches[1])){
    echo $matches[1]; // return "Car"
}

"The string will always have "Tmps" followed by something else."

You don't need a regular expression, in that case.

$result = substr($fieldName, 4);

If the first four characters are always the same, just take the portion of the string after that.

Given that the text you are looking in, contains more than just a string, starting with Tmps, you might look for the \w+ pattern, which matches any "word" char.

This would result in such an regular expression:

/Tmps(\w+)/

and altogether in php

$text = "This TmpsCars is a test";
if (preg_match('/Tmps(\w+)/', $text, $m)) {
    echo "Found:" . $m[1]; // this would return Cars
}