PHP Mysqli trigger a query one after other - php

Question

I am trying to fetch first id from one table and later after all the ids are fetched i am trying to fetch number of all that id's.
My Problem is
As i want to select the value of first query completion result
I am unable to trigger second query after the first query is completed both are triggerid at a time
First Query
$query ="SELECT * FROM abc WHERE xyz='xyz' And Standard='xyz' ";
$data=mysqli_query($mysqli,$query)or die(mysqli_error());
$ID = array();
while($row=mysqli_fetch_array($data)){
$ID[] = $row['ID'];
}
$IDall = "'" . implode("','", $ID) . "'";
Second Query
$query="SELECT mobno FROM euser WHERE UserId IN ($IDall)" ;
$data=mysqli_query($mysqli,$query)or die(mysqli_error());
$mobiles = array();
while($row=mysqli_fetch_array($data)){
$mobiles[] = $row['MobileNum'];
}
$mobilesStr = implode(',', $mobiles);
echo $mobilesStr;
}

Answer 1

Try this
SELECT mobno FROM euser WHERE UserId IN (SELECT ID FROM abc WHERE xyz='xyz' And Standard='xyz');

Answer 2

You don't need 2 queries. Only 1 is enough
SELECT mobno FROM euser WHERE UserId IN (
SELECT ID FROM abc ...
)

Answer 3

Try this
$query ="SELECT * FROM abc WHERE xyz='xyz' And Standard='xyz' ";
$data=mysqli_query($mysqli,$query)or die(mysqli_error());
if(mysqli_num_rows($data) > 0) {
$ID = array();
while($row=mysqli_fetch_array($data)){
$ID[] = $row['ID'];
}
$IDall = "'" . implode("','", $ID) . "'";
$query2="SELECT mobno FROM euser WHERE UserId IN ($IDall)" ;
$data2=mysqli_query($mysqli,$query2)or die(mysqli_error());
$mobiles = array();
while($row=mysqli_fetch_array($data2)){
$mobiles[] = $row['MobileNum'];
}
$mobilesStr = implode(',', $mobiles);
echo $mobilesStr;
}
In this the second query will execute when the first query has a result.