Select data from multiple tables from mysql?

Here is an implementation using mysqli and prevents injection for your $clientid where the id is a number grate then zero as AUTO_INCREMENT columns will never have a 0 value they start at 1

// as this is an int using inval will force it to be a valid whole number 
// basic SQL Injection Protection for a fixed id

$clientid = intval($_POST['client']);
if($clientid === 0){
    // it was not a valid number as an auto_increment field in mysql can never be 0
    die("invalid client");
}

$query="SELECT * FROM `pfs` 
    JOIN `pfssurety`  ON  pfssurety.clientid = pfs.clientid 
    JOIN `iso` ON  iso.clientid = pfs.clientid 
    JOIN `incometax` ON  incometax.clientid = pfs.clientid 
    WHERE pfs.clientid=$clientid";

$result= mysqi_query($query) or die("Query Failed");

while($row=mysql_fetch_array($result))
{
    echo $row['clientid'];
    echo $row['name'];
}

You meant to use backtique instead of single quote. Otherwise your table names are considered as normal string literal. Also, you are missing ON condition for all your JOIN clauses So your query

SELECT * FROM 'pfs'

Should actually be

SELECT * FROM `pfs`

Change your query to be

SELECT * FROM `pfs`
JOIN `pfssurety` ON condition
JOIN `iso` ON condition
JOIN `incometax` ON condition
WHERE clientid='$clientid'

Maybe you need to set the on in your join as something like this:

SELECT * FROM 'pfs'
JOIN 'pfssurety'  ON pfs.clientid=pfssurety.clientid
JOIN 'iso' ON pfs.clientid=iso.clientid
JOIN 'incometax' ON pfs.clientid=incometax.clientid
WHERE clientid='$clientid'

Y suposed that all tables have the clientid attribute.

• Instead of using the SELECT * you could use the enumeration of the different attributes you need

You are on the right track using JOIN. You need to specify the common column that the JOIN should be made on.

https://www.w3schools.com/sql/sql_join_left.asp

SELECT a.fieldname, i.fieldname, t.fieldname, p.fieldname FROM 'pfs' as a
LEFT JOIN 'pfssurety' as p ON p.clientid  = a.clientid 
LEFT JOIN 'iso' as i ON i.clientid  = a.clientid 
LFT JOIN 'incometax' as t ON t.clientid  = a.clientid 
WHERE a.clientid='$clientid'";

Also, you should escape your variables to prevent SQL Injection. At a minimum:

a.clientid= "' . mysqli_real_escape_string($clientid) . '"';

http://php.net/manual/en/mysqli.real-escape-string.php

Your code is vulnerable to SQL Injection, you should never use user input directly into your SQL queries. In your code the problem is here:

$clientid = $_POST['client']; // anyone could manipulate this field to inject malicious code
# ...
WHERE clientid='$clientid'";

Check what happens if the value for $_POST['client'] is: ' or 1 = 1;

Next as mentioned in one of the comments stop using deprecated methods, instead for example you can use mysqli. Here is an example of how to use mysqli with prepared statements to avoid SQL Injection:

$conn = new mysqli($servername, $username, $password, $dbname);
// Check connection
if ($conn->connect_error) {
    die("Connection failed: " . $conn->connect_error);
} 

$stmt = $conn->prepare('SELECT * FROM `pfs` JOIN `pfssurety` ON condition JOIN `iso` ON condition JOIN `incometax` ON condition WHERE clientid = ?');

$stmt->bind_param('i', $clientid);
$stmt->execute();
$stmt->close();
$conn->close();

A prepared statement is a feature used to execute the same (or similar) SQL statements repeatedly with high efficiency.

Compared to executing SQL statements directly, prepared statements have three main advantages:

• Prepared statements reduces parsing time as the preparation on the query is done only once (although the statement is executed multiple times)

• Bound parameters minimize bandwidth to the server as you need send only the parameters each time, and not the whole query

• Prepared statements are very useful against SQL injections, because parameter values, which are transmitted later using a different protocol, need not be correctly escaped. If the original statement template is not derived from external input, SQL injection cannot occur.

Finally one more thing worth mentioning, try not using * to fetch all columns, instead simply list the columns you need to get. Even if you need to get all columns there are good reasons why not to use *, but instead list all columns.