PHP - sum values except one in foreach

PHP - sum values except one in foreach

Ok I have this code
$var[0] = array('my_value=500,other_values,..');
$var[1] = array('my_value=700,other_values,..');
$sumValues = 0;
foreach ($vars as $key=>$var) {
$trek = $var->my_value;
if ( $key = 0 ) {
$sumValues = $some_external_value;
} else {
$sumValues = $sumValues + $trek[$key-1];
}
}
Expected returned values
$var[0] - $sumValues = 0
$var[1] - $sumValues = 500
$var[2] - $sumValues = 700
I want to sum up the values of $trek to be used for each $var as a value
only determined by the sum of the previous $var elements and not itself.
As always, thanks for any reply :)