Because overlapping periods may not be a simple == comparison, class periods must be split into start and end chunks and individually processed. Unfortunately, this makes for a loop-heavy block of code -- but it is unavoidable.
array_walk_recursive() is used to prepare your input data. The function, by design, only iterates the "leaf-nodes" of $sched -- this means that $v will never be an array, it will always be a time-span string. The function is also "depth-ignorant", so the only way properly modify the original $sched data is to make $v modifiable by reference (see: &$v).
Once the data is restructured, it is time to get loopy. A series of 3 successive loops traverse the "targeted" Class and Day subarrays for comparison. A further 2 loops are used to access the same Day subarrays (periods) of all other Classes. Earlier code builds implemented array_walk_recursive() again to reduce total loops, but this is not best practice because using a foreach loop with an early exit with break(2) saves iterations and improves efficiency.
Assuming that some classes may not have a day with period, I have baked in a Day check to ensure that my method doesn't cause Notices/Warnings based on missing Day subarrays.
Code: (Demo)
$sched=[
'Class A'=>[
'Day 1'=>['10:00-11:00','11:00-12:00','13:00-15:00','20:00-21:00'],
'Day 2'=>['12:00-13:00','14:00-15:00'],
'Day 3'=>['08:00-09:00','16:00-17:00','18:00-19:00'],
'Day 4'=>['10:00-11:00','12:00-14:00','14:00-16:00'],
'Day 5'=>['08:00-10:00','13:00-15:00'],
'Day 6'=>['08:00-09:00','10:00-11:00']
],
'Class B'=>[
'Day 1'=>['09:30-10:30','17:00-19:00'],
'Day 2'=>['08:00-09:00','11:00-12:00'],
'Day 3'=>['08:00-09:00','17:00-18:00','20:00-21:00'],
'Day 4'=>['08:00-09:00','14:00-16:00','17:00-18:00'],
'Day 5'=>['08:00-10:00','14:00-15:00'],
'Day 6'=>['10:00-11:00']
],
'Class C'=>[
'Day 1'=>['12:00-14:00']
]
];
// Preparation: Split the dashed time spans into chunked subarrays containing start and end times
// e.g. $sched['Class A']['Day 1'][['10:00','11:00'],['17:00','19:00']]
array_walk_recursive($sched,function(&$v){$v=explode('-',$v);});
// Comparisons:
foreach($sched as $class=>$days){
$other_classes=array_diff_key($sched,[$class=>'']); // same array without target class
foreach($days as $day=>$spans){
//echo "\n\n$class, $day";
foreach($spans as $period=>$span){
//echo "\nchecking {$span[0]} & {$span[1]}";
foreach($other_classes as $other_class=>$other_days){
if(isset($other_days[$day])){ // this avoids Notices/Warning about non-existent day in class
foreach($other_days[$day] as $other_period=>$other_span){ // compare other classes on same day
//echo "\nversus {$other_span[0]} & {$other_span[1]}";
if(($span[0]>=$other_span[0] && $span[0]<$other_span[1]) || ($span[1]<=$other_span[1] && $span[1]>$other_span[0])){
$result[]="Clash: $class,$day#".++$period.": {$span[0]}-{$span[1]} vs $other_class,$day#".++$other_period.": {$other_span[0]}-{$other_span[1]}";
break(2); // no need to further check this $span for clashes
}
}
}
}
}
}
}
echo implode("\n",$result);
